Explicitly cast values of different types

In case the warning -Werror=conversion is active with GCC, the warnings
about "conversion from A to B may change value" lead to a compilation
error. This explicitly convert the values to address these warnings.

src/catch2/catch_timer.cpp

@@ -30,7 +30,7 @@ namespace Catch {
         return static_cast<unsigned int>(getElapsedMicroseconds()/1000);
     }
     auto Timer::getElapsedSeconds() const -> double {
-        return getElapsedMicroseconds()/1000000.0;
+        return static_cast<double>(getElapsedMicroseconds())/1000000.0;
     }