Add user specifiable BCs to solvers (+ other small solver updates)

[lroberts36]

PR Summary

Add the option to define a SetBoundary function in a class defining a system of linear equations that will be used by the solvers instead of the default boundary conditions during boundary communication. Also moves some things common to all solvers to SolverBase and fixes a bug with the unique counters (previously, using multiple solvers within a single code was broken).

PR Checklist

• Code passes cpplint
• New features are documented.
• Adds a test for any bugs fixed. Adds tests for new features.
• Code is formatted
• Changes are summarized in CHANGELOG.md
• Change is breaking (API, behavior, ...)
  • Change is additionally added to CHANGELOG.md in the breaking section
  • PR is marked as breaking
  • Short summary API changes at the top of the PR (plus optionally with an automated update/fix script)
• CI has been triggered on Darwin for performance regression tests.
• Docs build
• (@lanl.gov employees) Update copyright on changed files

[pdmullen]

Changelog and style.

[Yurlungur]

LGTM