Generalize BVP classes to handle either real or complex numbers (Done by P.Palacios)

[PabloIbannez]

We have continue the PR made some months ago to generalize the BVP classes. Now we have defined some alises for both real or complex. For example:

using BatchedBVPHandlerReal = BatchedBVPHandler_impl; using BatchedBVPHandlerComplex = BatchedBVPHandler_impl<thrust::complex>;

We have modified the modules of DPStokes and Poisson to use this aliases. We have already run the test: misc/bvp and BDHI/DPStokes and they have passed succesfully.

Also the documentation has been updated.