Logging

[shivupa]

This implements a virtual Logging class which can be implemented by the user to log each iteration. This implements the proposed implementation "A" on #103.

The added tests are very simple and demonstrate an example of the derived logging class.

Closes #103

[codecov-commenter]

Codecov Report

Merging #131 (251d9e6) into master (904bbbe) will increase coverage by 0.2%. The diff coverage is 100.0%.

@@           Coverage Diff            @@
##           master    #131     +/-   ##
========================================
+ Coverage    92.1%   92.3%   +0.2%     
========================================
  Files          45      46      +1     
  Lines        2140    2229     +89     
========================================
+ Hits         1971    2059     +88     
- Misses        169     170      +1     

Impacted Files	Coverage Δ
include/Spectra/GenEigsComplexShiftSolver.h	100.0% <ø> (ø)
include/Spectra/GenEigsRealShiftSolver.h	100.0% <ø> (ø)
include/Spectra/SymEigsShiftSolver.h	100.0% <ø> (ø)
include/Spectra/SymGEigsShiftSolver.h	94.4% <ø> (ø)
include/Spectra/SymGEigsSolver.h	100.0% <ø> (ø)
include/Spectra/DavidsonSymEigsSolver.h	100.0% <100.0%> (ø)
include/Spectra/GenEigsBase.h	86.1% <100.0%> (+0.8%)	:arrow_up:
include/Spectra/GenEigsSolver.h	100.0% <100.0%> (ø)
include/Spectra/JDSymEigsBase.h	90.7% <100.0%> (+1.6%)	:arrow_up:
include/Spectra/LoggerBase.h	100.0% <100.0%> (ø)
... and 6 more

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[shivupa]

Thanks for the quick review and the advice!

[shivupa]

@JensWehner If you could take another pass I'd appreciate it!