Add .dump() method

[Adamtaranto]

Similar to dump in KMC_tools, write kmer:count pairs to a text file.

Could add two options for this:

• .dump_hash() for hash:count pairs
• .dump() for kmer:count pairs

The second option should throw an error if no hash:kmer map is stored in the KmerCountTable object.

Should we support sorting output on either kmer or count?

[Adamtaranto]

Check out Pariter for parallel queries that preserve input / output order.

[ctb]

(I've used rayon extensively - ref also #22)

[Adamtaranto]

Just adding .dump_hash() for now until we have a solution for #21.

Default sort on counts then keys. Option to sort on keys.

@ctb For writing output, should we:

1. Take a outfile arg and handle in rust?, or
2. Yield output lines and handle the writing in Python?

2 is easier to write tests for, 1 is probably easier for users.