Updated domain variables & bugfixes

Changed domain variable seeding to follow statistical distributions with easier scaling
Fixed dataview for irradiance metadata (bugfix)
Allows 0 residual during calculations (bugfix)

Round-tripped models & fixed realtime plot

Minor bugfixes for CLI level use

Updated to python3, improved performance for stiff equations

Since the core dependency of fipy is now usable directly in python3, microbenthos is now considered python3-only software (> python 3.6)

Time-stepping heuristics were improved to deal with numerical instabilities for stiff differential equations. API remains unchanged, though there are behavioral changes in running Simulation.run_timestep and Simulation.evolution.

Other small improvements in the submodules.

JOSS review completed

Peer-reviewed and approved version of microbenthos for Journal of Open Source Software