Implementation of TimeIndependentPDESolver.

- used variadic templates to avoid code bloat
- added execHost static method to avoid nvcc warnings

- separate data types for input arrays of binary operations
  (necessary for the equality comparison of two arrays of different
  type)
- added separate template parameter for ResultType, which defaults to
  the RealType
  (the goal is to allow sum of bool vectors into int instead of bool)