Modify how modulo is calculated in Normal and Scientific mode. (#412)

## Fixes #111

> The modulo operator on this calculator gives the result that is different to the most used calculators.

The current `modrate` function is the equivalent of rem(...)/remainder(...), not mod(...)/modulo(...) available in some popular Math apps. 

### Description of the changes:
- rename `modrate` in `remrate` to be more accurate.
- add `modrate`, calculating modulo similarly to Matlab, Bing, Google calculator, Maxima, Wolfram Alpha and Microsoft Excel 
- Add `RationalMath::Mod` using `modrate` as an alternative to `Rational::operator%` using `remrate`
- Add a helper `SIGN` to retrieve the sign of a `Rational`.
- modify `CalcEngine` to use `modrate` in Normal and Scientific mode and `remrate` in Programmer mode.

### How changes were validated:
- manually and unit tests added

src/CalculatorUnitTests/CalcEngineTests.cpp

@@ -13,7 +13,7 @@ using namespace Microsoft::VisualStudio::CppUnitTestFramework;
 
 static constexpr size_t MAX_HISTORY_SIZE = 20;
 
-namespace CalculatorUnitTests
+namespace CalculatorEngineTests
 {
     TEST_CLASS(CalcEngineTests)
     {