Add sliding boundary conditions in push/pull

[balbasty]

Sliding boundary conditions are specific to displacement fields: they use a mixture of DCT2 and DST2 boundaries, with the DST2 dimension being different in each channel.

Here's an example in 3D:

• Channel [0] = displacement along X --> bound = (DST2, DCT2, DCT2)
• Channel [1] = displacement along Y --> bound = (DCT2, DST2, DCT2)
• Channel [2] = displacement along Z --> bound = (DCT2, DCT2, DST2)

They must be implemented in pushpull_common.cpp, in different flavours:

• 2D and 3D generic (any interpolation order)
• 2D and 3D isotropic linear
• 2D and 3D isotropic nearest