Refactor add_nested to join_nested

[hombit]

Currently, the only supported way to add nested columns is by packing them by index and assigning them to NestedFrame with the same index values. However, as was found during the LSDB integration (astronomy-commons/lsdb#368, astronomy-commons/lsdb#367), it is important to pack by the value column instead of index.

Additionally, it would be nice to make the interface closer to pandas’ join by supporting on and how arguments.

The following steps are proposed:

• Rename add_nested to nested_join
• Add how argument to add_nested() #115
• Add on argument for pack() #117
• Add on argument to add_nested #116

[hombit]

Everything but renaming is implemented in v0.3.0