add insitu retrieval for cmems nrt

[veenstrajelmer]

cmems_obs-ins_glo_phybgcwav_mynrt_na_irr might also be good to add, but is much more messier. Requires filtering and merging of stations, so not good as a default datasource. Not yet subsettable via API. If added, we need to filter the file_name column containing /history/TG/ and additionally drop MO_TS_TG_ANDRATX.nc to avoid time-varying lat/lon variables. For the rest, a comparable approach can be used as for the cmems-my dataset