Pre-release: normalisation model, plot improvements, bug fixes

This is the last pre-release before we introduce cell2location based on pyro and scvi-tools. The main purpose of this release is fixing package versions and dependencies for pymc3 implementation of models - which helps to provide docker image that reproducibly works with older models and older user interface (run_cell2location / run_regression / etc).

This pre-release includes 1) the model which can normalise within experiment variation in RNA detection sensitivity, 2) improvements to data visualisation, 3) major refactoring of how models are organised, 4) bug fixes (no critical bugs).