adding GPU support for LTS

[danielpeter]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 16.38225% with 245 lines in your changes missing coverage. Please review.

Project coverage is 49.45%. Comparing base (e0cfa3b) to head (b197760).
Report is 5 commits behind head on devel.

Files with missing lines	Patch %	Lines
src/specfem3D/lts_setup.F90	29.50%	86 Missing ⚠️
src/specfem3D/lts_assemble_MPI_vector.F90	4.70%	81 Missing ⚠️
src/gpu/specfem3D_gpu_cuda_method_stubs.c	0.00%	56 Missing ⚠️
src/specfem3D/lts_global_step.F90	27.27%	8 Missing ⚠️
src/specfem3D/prepare_gpu.f90	0.00%	5 Missing ⚠️
src/specfem3D/assemble_MPI_vector.f90	0.00%	3 Missing ⚠️
...3D/compute_forces_viscoelastic_calling_routine.F90	0.00%	2 Missing ⚠️
src/specfem3D/lts_newmark_update.F90	66.66%	2 Missing ⚠️
src/specfem3D/compute_add_sources_acoustic.f90	0.00%	1 Missing ⚠️
src/specfem3D/fault_solver_dynamic.f90	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            devel    #1803      +/-   ##
==========================================
- Coverage   49.58%   49.45%   -0.13%     
==========================================
  Files         272      272              
  Lines       63235    63449     +214     
==========================================
+ Hits        31353    31378      +25     
- Misses      31882    32071     +189     

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