numeric stability and minor improvements to estimators

• fixes singular matrix issues in most cases
• catches overseen closed form solutions in estimator init

minor changes to numpy backend

v0.7.3

Merge branch 'dev' of https://github.com/theislab/batchglm into dev

tf2 compatibility and numpy backend release

• prepares tf2 backend
• introduces numpy backend for NB distribution

improvement in constraint and rank handling

v0.6.8

fixed constraint unit test

changes according to diffxpy modifications

v0.6.7

fixed bug that division was forced as numpy even though x could be cs…

Removed remaining xarray references

v0.6.6

removed remaining xarrax references

Fixed missing reference in data api

v0.6.5

changed default estimator batch size to 512

Adapated typing interface

v0.6.4

relaxed numpy and tf requirements

Removed xarray dependency

Removed dependency in batchglm and adapated interface to diffxpy slightly.

Upload to pypi

adapted to anndata not exporting anndata.base anymore

Adapting according to discussion at https://github.com/theislab/anndata/issues/174.