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/CalcManager/CEngine/scioper.cpp

@@ -78,7 +78,7 @@ CalcEngine::Rational CCalcEngine::DoOperation(int operation, CalcEngine::Rationa
         case IDC_DIV:
         case IDC_MOD:
         {
-            int iNumeratorSign = 1, iDenominatorSign = 1, iFinalSign = 1;
+            int iNumeratorSign = 1, iDenominatorSign = 1;
             auto temp = result;
             result = rhs;
 
@@ -107,20 +107,30 @@ CalcEngine::Rational CCalcEngine::DoOperation(int operation, CalcEngine::Rationa
 
             if (operation == IDC_DIV)
             {
-                iFinalSign = iNumeratorSign * iDenominatorSign;
                 result /= temp;
+                if (m_fIntegerMode && (iNumeratorSign * iDenominatorSign) == -1)
+                {
+                    result = -(Integer(result));
+                }
             }
             else
             {
-                iFinalSign = iNumeratorSign;
-                result %= temp;
-            }
+                if (m_fIntegerMode)
+                {
+                    // Programmer mode, use remrat (remainder after division)
+                    result %= temp;
 
-            if (m_fIntegerMode && iFinalSign == -1)
-            {
-                result = -(Integer(result));
+                    if (iNumeratorSign == -1)
+                    {
+                        result = -(Integer(result));
+                    }
+                }
+                else
+                {
+                    //other modes, use modrat (modulus after division)
+                    result = Mod(result, temp);
+                }
             }
-
             break;
         }