Update action and tests for the Eigenvalue executioner

[smpark7]

Description

Closes #177
This PR transitions Moltres from the old NonlinearEigen/InversePowerMethod executioner implementation to the new Eigenvalue executioner implementation in MOOSE for solving k-eigenvalue problems. The old implementation is deprecated and will eventually be removed from MOOSE. All relevant tests and documentation are updated to reflect the transition to the new implementation.

A k-eigenvalue problem may be formulated as $Lx = \frac{1}{k} Fx$, where the $F$ operator comprises of neutron source terms and the $L$ operator comprises of the streaming, collision, and scattering terms.

Unlike the old implementation which requires a separate EigenKernel class for the $F$ operator, the new implementation uses the regular Kernel class with the extra_vector_tags parameter to indicate whether it belongs in $F$ (otherwise $L$).

This PR also refactors NtAction to reduce code duplication. NtAction is an Action object which streamlines the process of creating Moltres input file by automatically setting up various Kernels, BCs, etc. required in a neutronics simulation.

Changes

• Remove deprecated EigenKernels in src/ & include/, i.e., CoupledFissionEigenKernel, PrecursorEigenSource, SelfFissionEigenKernel
• Remove documentation files in doc/ associated with the EigenKernels
• Update NtAction and PrecursorAction to use the new EigenProblem implementation
• Update test input and output files in tests/ to use the new EigenProblem implementation
• Update input files in problems/ to use the new EigenProblem implementation
• Update the instructions on how to run the 2021-cnrs-benchmark input files and the expected computational requirements
• Update tutorial input files in tutorials/ to use the new EigenProblem implementation
• Update the input syntax and eigenvalue tutorials to use the new EigenProblem implementation
• Fix wrong int signedness in ElmIntegFissPostprocessor and ElmIntegFissNtsPostprocessor in src/ and include/
• Refactor NtAction to reduce code duplication

Impact

• Brings Moltres up to date with the new Eigenvalue executioner implementation in MOOSE
• Improved code cleanliness in NtAction

[yardasol]

Hi @smpark7,

Good work on this code update and refactor. You did a good job updating the docpages. I'm going to approve, but am curious why many int variables were changed to unsigned int.

[smpark7]

Thanks for the review @yardasol! I changed the parameters to unsigned int for consistency. All other occurrences of those parameters in Moltres are unsigned ints. Feel free to merge if you (re)approve.

[yardasol]

Good work @smpark7!