Feature complete Series accessor

[znicholls]

The motivation

We have accessors for DataFrames, but haven't ported a lot of this functionality across to Series (or Index) accessors.

The proposed solution

Add the following features:

Series

• ensure_index_is_multiindex (Add ensure is multiindex accessors #36)
• fix_index_name_after_groupby_quantile (More series pieces #37)
• groupby_except (More series pieces #37)
• mi_loc (More series pieces #37)
• to_category_index (More series pieces #37)
• update_index_levels (Add more missing features from accessors #39)
• update_index_levels_from_other (Add more missing features from accessors #39)

Index

• ensure_is_multiindex (Add ensure is multiindex accessors #36)
• update_levels (Add more missing features from accessors #39)
• update_levels_from_other (Add more missing features from accessors #39)

Alternatives

Don't provide accessors for these

Additional context