pmgbergen/porepy: PorePy version 1.7.0

This PR provides several major updates of PorePy. The most important changes are: The way equations are specified and solved is completely reworked. The new Model classes provide a flexible way to set up simulations, with extreme modularity to allow for manipulation of individual terms in equations. These classes are now the new recommended way of defining simulation models in PorePy. Documentation is much improved, with updated tutorials as well as updated docstrings that cover a large part of the code. Bugs and other shortcomings have been corrected throughout the code base. From now on, our aim is to move to a more regular schedule of 2-3 yearly releases. The next release of PorePy is tentatively scheduled for September 2023. New Contributors @Yuriyzabegaev @wmboon @pschultzendorff @solveigste @mariusnevland @IngridKJ @taj004 Full Changelog: https://github.com/pmgbergen/porepy/compare/v1.6.0...v1.7.