WIP: Combine KmerCountTables with Add()

[Adamtaranto]

• [x] Add counts from a second KmerCountTable
• [x] Add hash_to_kmer records from a second KmerCountTable
• [x] Add tests
• [x] Handle case adding table to itself

Closes #62

[Adamtaranto]

Not sold on having a subtract method, might get rid of it.

[Adamtaranto]

Ditched the subtract function.

Need to decide if we should allow adding a table to itself - this creates issues around borrowing the same object multiple times and is generally a bit of a pain.

Maybe just raise error if trying to add table to itself?

[Adamtaranto]

I'm giving up on trying to check if self and other are the same table. Just let it die with borrow error.

[ctb]

hah! over in https://github.com/oxli-bio/oxli/pull/83 I created a _merge method that does the same thing. let me look...

[ctb]

I am wondering if we want to avoid doing merges in parallel fashion? The locking is going to add a lot of overhead I think.

[Adamtaranto]

Should I fully remove parallelisation from the function?