v1.0: reindexing bug fix

- when data and variogram have unequal indices, nan are inserted for the
missing steps. These nans are floats rather than strings, this caused
errors when floats were used where strings were expected.

- That has been fixed by reindexing the vg_ser and cast as str.

- If index is obj then indices of vg_ser and data_df are cast to string.
This allows for a consistent datatype and behavior.