Add new dunder methods and attributes

[Adamtaranto]

New dunder methods for more Pythonic interaction:

• Make KmerCountTable iterable, returns (hash,count) tuples.
• Add __setitem__, Set values using the indexing syntax obj[key] = value
• Add __getitem__, Query an object to using the indexing syntax obj[key]
• Add len() method, returns number of keys in KmerCountTable.

New attributes:

• .version, returns oxli version that object was created with (will use later when serialising objects)
• .consumed, returns total number of bases consumed using either count() or consume()
• .sum_counts, returns sum total of all counts in table.

Note: Have added "toml" package as a conditional dependency for testing .version. It is in pyproject.toml under optional dependencies. Will be installed if installing oxli from repo via pip install -e .[test]

Closes #20

[Adamtaranto]

Was using toml library because I didn't know about tomllib. Will look into it.

[Adamtaranto]

@ctb Looks like tomllib was introduced in Python 3.11.

Do we want 3.11 to be the min version required for this package? or keep the external toml import?

I'm going to leave it for now and merge this branch. Can update later.