Adding `--genome-size` flag to override auto-calculated genome size

[danejo3]

The purpose of this PR is to resolve #28 .

Sometimes, when YEAT auto calculates the genome size with MASH, the value can be off or if the user knows exactly the genome size, it's better to go with that number when appropriate.

In this PR, users can call the --genome-size flag to override YEAT's auto calculated size with the user's input.

[danejo3]

Okay! This is a relatively simple PR and ready to ship!

[standage]

This is a relatively simple PR

Indeed, but not so simple that we can skip running the tests. :smiley:

SyntaxError in line 90 of yeat/Snakefile:
invalid syntax

[danejo3]

oops