moved cuda openmp into own subsystem

[JensWehner]

The Cuda Openmp things are moved into a separate class, so that threecenter_gwbse only focuses on the physics and not on the scheduling.

[JensWehner]

@votca-bot changelog: Refactored CUDA-OpenMP integration

[codecov]

Codecov Report

Merging #571 into master will increase coverage by 0.0%.
The diff coverage is 96.6%.

@@          Coverage Diff           @@
##           master    #571   +/-   ##
======================================
  Coverage    57.3%   57.3%           
======================================
  Files         312     313    +1     
  Lines       32391   32394    +3     
======================================
+ Hits        18565   18568    +3     
  Misses      13826   13826           

Impacted Files	Coverage Δ
include/votca/xtp/threecenter.h	80.0% <ø> (ø)
src/libxtp/gwbse/gwbse.cc	0.0% <0.0%> (ø)
src/libxtp/bsecoupling.cc	64.3% <100.0%> (ø)
src/libxtp/openmp_cuda.cc	100.0% <100.0%> (ø)
src/libxtp/threecenter_gwbse.cc	100.0% <100.0%> (ø)

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[felipeZ]

This is so much cleaner and easier to understand. Great Job @JensWehner!