Overview of main changes

This PR contains several major changes that require updates to runscripts:

The data structures for the mixed-dimensional grid (formally the GridBucket) has been thoroughly overhauled, see #673 (comment) for a description of the new framework.
The interface for export to Paraview has been rewritten, with the new version being more flexible, cleaner and a lot faster. An overview of how to interact with the new structure is given in https://github.com/pmgbergen/porepy/blob/develop/tutorials/exporter.ipynb.
The tutorials have been thoroughly overhauled.

Other notable changes:

Typing coverage has been much extended.
Work on an improved
Docker files have been updated, these will be further improved in the coming months.
A test for point-in-polyhedron test has been implemented, removing the need to download an external file and thereby simplifying the installation procedure.
Several bugfixes and performance improvements.

It is becoming clear that the new Automatic Differentiation framework is the way to go for assembling multiphysics systems. To reduce the burden of maintenance, it has been decided to deprecate the Assembler, together with several associated methods and classes, with deletion scheduled for the second half of 2022. For further details, see #673 (comment) (towards the end of the description of the PR).
Support for Python 3.7 has been dropped.

The following users made their first contribution to PorePy:

Also contributing to this release were:

Full Changelog: v1.5.0...v1.6.0