PyPi package and automated species data

New features

• --species (or -s) flag to specify one of a few built-in species data. This negates the need for specifying tRNA references, tRNAscan out files, and mitochondrial tRNA sequences
• Species files for which there isn't built in data can still be specified using -t, -o and -m
• Package now available on PyPi and installable using pip
• mods/readthroughTable.csv is now and additional which gives information on the proportion of reads that stop at a given site relative to the total reads at that site (as opposed to RTstopTable.csv which gives the proportion of total reads for the tRNA that stop at a site)