Naming inconsistency File/Variable for putting attribute

For adding a variable we use [`add_attribute`](https://docs.rs/netcdf/latest/netcdf/struct.FileMut.html#method.add_attribute) for `FileMut`, but [`put_attribute`](https://docs.rs/netcdf/latest/netcdf/struct.VariableMut.html#method.put_attribute) for `VariableMut`. This should be `put_attribute` for both.

In addition other namings should be considered to more closely match `netcdf-c`, e.g. `add_type` -> `nc_def_dim`/`def_type`