Various fixes

[bosd]

Cherry-picks from: https://github.com/foarsitter/camelot/pull/72

Below is documentation on each of the changes

1. core.py:

*Table.to_excel method bug-fix: With the installed version of pandas in pypdf_table_extraction, the pd.ExcelWriter object no longer has a write method. I've updated this method accordingly to write the file in the recommended manner.

• Linting changes: In my personal usage, I found the type hinting of the original camelot package left a lot to be desired. My personal use case made the Table and TableList objects most important, so I've added type hinting to these methods that enable type checkers and linters to provide useful hints when using these objects or functions that return them. For example, I used to get linting errors about iterating through a TableList, whereas now I don't! The linter is also able to tell that the objects within the iterable are Tables. Just a small QOL change that went a long way in my use-case, and likely will for others.

2. lattice.py:

• Ensuring indices has elements: This is a small but mighty change! We found errors consistently at this line. After returning from lattice.get_table_index, there is no check for whether the indices list has any elements. This slight modification does the indices reduction if the indices list has no elements.