Implementation of nonlinear diffusion acceleration

[JessicaRehak]

This pull requests implements nonlinear diffusion acceleration.

Nonlinear diffusion acceleration is a post-iteration sub-process of type GetScalarFluxFromFramework that uses a framework identical to the top level framework but uses the drift-diffusion formulation. The angular flux is provided by the top level angular solve.

This also adds a multi-slab test problem from Picca 2016 for testing.

[JessicaRehak]

Issues that will need to be addressed:

• Need to create an enum that identifies locations for possible sub-processes to be installed in a class. This can then be used an abstracted class that has a template parameter that identifies locations for subroutines and then accesses a map to get the subprocesses when called.
• Need to identify how to abstract out and generalize the struct that holds the data for the NDA algorithm for other subprocesses so the framework parameters struct doesn't just keep growing in size.

[codecov]

Codecov Report

Merging #209 (50d9e44) into dev (05a6369) will decrease coverage by 0.08%.
The diff coverage is 95.77%.

@@            Coverage Diff             @@
##              dev     #209      +/-   ##
==========================================
- Coverage   98.60%   98.51%   -0.09%     
==========================================
  Files         204      206       +2     
  Lines        4001     4050      +49     
==========================================
+ Hits         3945     3990      +45     
- Misses         56       60       +4     

Impacted Files	Coverage Δ
src/domain/definition.h	100.00% <ø> (ø)
src/domain/definition_i.h	100.00% <ø> (ø)
...rc/domain/finite_element/finite_element_gaussian.h	100.00% <ø> (ø)
src/domain/mesh/mesh_cartesian.cpp	98.91% <ø> (ø)
src/formulation/updater/diffusion_updater.hpp	100.00% <ø> (ø)
src/formulation/updater/saaf_updater.h	91.66% <ø> (ø)
src/framework/builder/framework_builder.hpp	92.10% <ø> (ø)
src/framework/builder/framework_builder_i.hpp	100.00% <ø> (ø)
src/framework/builder/framework_validator.hpp	100.00% <ø> (ø)
src/framework/builder/framework_validator_i.hpp	100.00% <ø> (ø)
... and 14 more

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[JessicaRehak]

Check failed due to running out of travis credits. Closes #5.