Allow to reduce compilation overhead

[ddemidov]

by disabling unused components in runtime interface. This should help with 'compiler is out of heap space' kinds of errors during compilation with MSVC.

For now it is possible to disable

• multicolor_gauss_seidel,
• parallel_ilu0,
• spai1, and
• chebyshev

relaxation types by correspondingly defining

• AMGCL_RUNTIME_DISABLE_MULTICOLOR_GS,
• AMGCL_RUNTIME_DISABLE_PARALLEL_ILU0,
• AMGCL_RUNTIME_DISABLE_SPAI1, and
• AMGCL_RUNTIME_DISABLE_CHEBYSHEV,

preprocessor definitions.

CC @RiccardoRossi.

[codecov-io]

Codecov Report

Merging #45 into master will decrease coverage by 0.03%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
- Coverage   82.47%   82.44%   -0.04%     
==========================================
  Files          71       71              
  Lines        4366     4368       +2     
  Branches      742      742              
==========================================
  Hits         3601     3601              
- Misses        707      709       +2     
  Partials       58       58

Impacted Files	Coverage Δ
tests/test_solver.hpp	98.52% <ø> (ø)	⬆️
amgcl/runtime.hpp	96.24% <0%> (-0.73%)	⬇️
amgcl/relaxation/runtime.hpp	89.04% <75%> (-1.24%)	⬇️

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