Integrate creation of offsets and parquet files

Currently the kerchunk offsets are being created separately but in order to complete the workflow generating these and the parquet files can be incorporated into the library. The following would be nice to have:

# create offsets for the given tar,
# append "offsets" to the filename
zarranalyzer.kerchunk("./ngc4008_P1D_0.tar")

# create the reference files
zarranalyzer.create_parqs("./ngc4008_P1D_0.tar.offsets", "ngc4008_P1D_0.parq") 

Then these can run immediately after the dispatch is done.