Switch to miniforge

[bernt-matthias]

Due to the change in Anaconda Inc's TOS many institutions do not allow to use conda's default channel. Since miniconda uses default (e.g. for setting up the base environment and its also included in the default list of channels) we also using miniconda is forbidden.

As a drop in replacement miniforge should be used.

The main point is the Makefile, but also a few tutorials could be updated.