First checkin
Tidies code and adds proper assertion into the unit test
Refactors Permutation to NumberPermutation
Adds CalculationPermutation created from NumberPermutation.
Adds calculation and sanitises test cases.

Removes Solution class as the calculation permutation will suffice.
Parallelises the stream searching for solutions
Extracts Calculator and unit test harness.
Refactors CalculationPermutation to Calculation, adds main class and method