Thomas/seissolxdmfwriter group filter

[Thomas-Ulrich]

allow group filter on mesh:

seissol_output_extractor tmp/Myanmar_gebco_down2_with_fault_m11_bm_1.xdmf --regionFilter "0,2" --var group --time "0"

now works.
Apply isort.

[davschneller]

LGTM

[davschneller]

At some point it might be useful to make this section e.g.

possible_varnames = ['fault-tag', 'locationFlag', 'group']
region_varname = None
for candidate in possible_varnames:
    if candidate in available: region_varname = candidate
if region_varname is None:
    raise ValueError("...")

to avoid some typos when adding new region variables (e.g. if someone were to look at the global-ids for whatever reason?)