Use dataclass for data stored in load package

[rwood-97]

Currently in the load package we store data as nested dictionaries. This is frustrating as you can get key errors.
It would be good to use dataclasses to store data whenever possible in MapReader. This mostly applies to the text spotting (see #533 ) and load subpackages.

For load subpackage:

• Use dataclass for storing info about patches and parent images