V2 project (hack_DC_TGV branch): implementation of Unit Tests

[pbartholomew08]

The aim is to be able to test automatically derivatives/interpolations/filters regularly (especially after an update of the code)

-UT for each first derivative subroutine in x y and z directions
-UT for each second derivative subroutine in x y and z directions
-UT for each interpolation subroutine in x y and z directions
-UT for each filter subroutine in x y and z directions
-UT for the Poisson solver

Each UT will have to work for all possible sets of boundary conditions. The idea is to define a cos or a sin function to be derived/interpolated, and compare the results with the analytical solution. An error threshold would have to be defined for automatic testing.

See other issue raised by Thibault for more information.