When using C++11, comparison operators are already templated to take
anything that can be explicitly converted to double, but constructor
took only doubles. This lead to warnings when an `Approx` was
constructed from floats, which was problematic for some users.
Since just adding float constructor would be a large breaking change, as
suddenly `Approx( 1 )` would become ambiguous, I added a templated
constructor that will take anything that is explicitly convertible to
double. This has the added benefit of allowing constructing `Approx`
instances from instances of strong typedefs, ie allowing
`calculated_temp == Approx( known_temp)`.
Closes#873