Test for from_single_mtx
The merge request for https://gitlab.gwdg.de/loosolab/software/loosolab_sc_rna_framework/-/issues/14.
I restructured the code within assemblers, such that we now have "from_quant", which calls "from_single_starsolo" per sample. Internally "from_single_starsolo" calls "from_single_mtx" to assemble the anndata object.
Preliminarily, the "from_single_mtx" is tested using the data in the new "data" dir, containing any data used for unit testing.