One face region

[IvanaEscobar]

Reviving #335

Old patches used for applications of domains contained in a single face.

Helps to:

• set offset_vars when file_metadata only has a single variable
• use find_concat_dim_facet if DataArray input doesn't contain a face dimension
• use llc regions that aren't starting on facet0, reads nz not the first facet which works in existing domains defined in fxn get_extra_metadata

[IvanaEscobar]

This PR needs to be fixed to work with tests!