This commit addresses several issues. It essentially forbids the use of
the GPU kernel, which become obsolete and caused problems. But it
does not complete remove the related code, nor does it forbid from
explicitly selecting the GPU kernel. However, if the user does select
it, the warning will be issued and the GENERIC kernel would be used
instead. In the more details:
* Commentin out operations in the GPU kernel, which do not compile with
  CUDA 10.1. This makes the kernel deffinitely not ussable (but it was
  true even before)
* removing the gpu_tridiag_band option, sincie the tridiag->banded routine
  is actually not ported to GPU at all. This step will thus always be
  run on the CPU
* removing the gpu_trans_ev_tridi_to_band option, since the GPU version
  of this step cannot run without the GPU kernel and it is not usable.
  This step will thus also be performed on the CPU
* modifying REAL_GPU_KERNEL_ONLY_WHEN_GPU_IS_ACTIVE and
  COMPLEX_GPU_KERNEL_ONLY_WHEN_GPU_IS_ACTIVE such that the GPU kernel is
  not considered during the autotuning

* TODO however, the GPU kernel can still be enforced by the user. In
  this case, during the calculation, a warning is issued and the kernel
  is switched to the GENERIC one. This should be improved and there
  should not even be the possibility to choose the GPU kernel at the
  begining.