implementing ratio<num,den,exp> which replaces ratio<num,den>

https://github.com/mpusz/units/issues/14

This "works", as in it passes all static and runtime tests.
However quite a few of the tests have been "modified" to make them pass. Whether
this is legitimate is debatable and should be the source of some thought /
discussion.

1. many of the static tests and some of the runtime tests have had the input
ratios of the tests modified in the following way. eg ratio<3,1000> =>
ratio<3,1,-3>. ie they have been "canonicalised".

There are obviously an infinite number of ratios which represent the same
rational number. The way `ratio` is implemented it always moves as "many powers
of 10" from the `num` and `den` into the `exp` and that makes the `canonical`
ratio.

Because these are all "types" and the lib uses is_same all over the place, only
exact matches will be `is_same`. ie ratio<300,4,0> !is_same ratio<3,4,2> (the
latter is the canonical ratio). This is perhaps fine for tests in the devlopment
phase, but there may be a need for "more forgiving" comparison / concept of
value equality. One such comparison which compares den,num,exp after
canonicalisation is the constexpr function `same` as defined at top of
`ratio_test.cpp`. We may need to expose this and perhaps add even more soft
comparisions.

2. In the runtime tests it is "subjective" how some resukts should be
printed. There is the question of "how exactly to format certain ratios". eg
omit denominators of "1" and exponents of "0". However before even addressing
these in detail a decision needs to be made about the general form of
"non-floating-point-converted" ratios which do not map exactly to a "Symbol
prefix".

Arguably these are "relatively ugly" whatever we do, so we could just
go for an easily canonicalised form. An example is:

-        CHECK(stream.str() == "10 [1/60]W");
+        CHECK(stream.str() == "10 [1/6 x 10⁻¹]W");

Which of thses is "better"? Is there a "third", better form?  It's not obvious.

My opnion is: Both of 1&2 are fine for now, unless we think they go down the
wrong avenue, and can be "perfected later"? ie we can expose a softer version of
ratio based equality, and decide on canonical way of printing ratios (as far as
that is actually a very useful output form, compared with decimal, scientific or
engineering notation).

test/unit_test/static/unit_test.cpp

@@ -30,12 +30,12 @@ using namespace units;
 struct metre : named_unit<metre, "m", si::prefix> {};
 struct centimetre : prefixed_unit<centimetre, si::centi, metre> {};
 struct kilometre : prefixed_unit<kilometre, si::kilo, metre> {};
-struct yard : named_scaled_unit<yard, "yd", no_prefix, ratio<9'144, 10'000>, metre> {};
+struct yard : named_scaled_unit<yard, "yd", no_prefix, ratio<9'144, 1, -4>, metre> {};
 struct foot : named_scaled_unit<foot, "ft", no_prefix, ratio<1, 3>, yard> {};
 struct dim_length : base_dimension<"length", metre> {};
 
 struct second : named_unit<second, "s", si::prefix> {};
-struct hour : named_scaled_unit<hour, "h", no_prefix, ratio<3600>, second> {};
+struct hour : named_scaled_unit<hour, "h", no_prefix, ratio<36, 1, 2>, second> {};
 struct dim_time : base_dimension<"time", second> {};
 
 struct kelvin : named_unit<kelvin, "K", no_prefix> {};
@@ -46,8 +46,8 @@ struct dim_velocity : derived_dimension<dim_velocity, metre_per_second, exp<dim_
 struct kilometre_per_hour : deduced_unit<kilometre_per_hour, dim_velocity, kilometre, hour> {};
 
 static_assert(std::is_same_v<downcast<scaled_unit<ratio<1>, metre>>, metre>);
-static_assert(std::is_same_v<downcast<scaled_unit<ratio<1, 100>, metre>>, centimetre>);
-static_assert(std::is_same_v<downcast<scaled_unit<ratio<yard::ratio::num, yard::ratio::den>, metre>>, yard>);
+static_assert(std::is_same_v<downcast<scaled_unit<ratio<1, 1, -2>, metre>>, centimetre>);
+static_assert(std::is_same_v<downcast<scaled_unit<ratio<yard::ratio::num, yard::ratio::den, yard::ratio::exp>, metre>>, yard>);
 static_assert(std::is_same_v<downcast<scaled_unit<ratio_multiply<typename yard::ratio, ratio<1, 3>>, metre>>, foot>);
 static_assert(std::is_same_v<downcast<scaled_unit<ratio_divide<typename kilometre::ratio, typename hour::ratio>, metre_per_second>>, kilometre_per_hour>);