Merge OXRX, NUTRX, and PLANK class approach into LimnoTech develop

[aufdenkampe]

This PR is intended for long-term tracking of:

• commits on the new class approach to water quality modules, as described by Complete OXRX, NUTRX, PLANK PHCARB modules w/ class-based code respec/HSPsquared#58;
• differences with RESPEC's develop branch (via LimnoTech's develop branch, for assistance in identifying merge conflict earlier than later; and
• conversations about when it is ready to merge and issue a PR to RESPEC toward a new release.

This PR builds on the First set of Wq updates from tmr: no import errors! #38 PR.

We don't plan on merging this PR until we have this code working.

cc: @tredder75, @ptomasula, @PaulDudaRESPEC