Function to summarize bases at a given position in a fasta file

Added a function to take a fasta file with sequences and a position and return information about which bases / amino acids are found at that position.
Sorry to do it on my master, I got over exited...
I think it would be good to add a `--type` command line arg, so we can pass in a DNA or AA fasta file.

We already have a command in `bin` that takes a type, and I suspect it has a bug in it (since we now pass the read class not a type string) so maybe you could copy that and fix the bug.