Detect binaries automatically if they are in the PATH

[guma44]

Path to Salmon binary is now hardcoded. It would be better when PISCES would check if salmon is in the PATH eg. like here https://stackoverflow.com/questions/377017/test-if-executable-exists-in-python.

[mdshw5]

I think this is a great idea, but would also like to fall back on bundling a version of salmon that works with PISCES. Some of the older versions (< 1.0) are incompatible with genome-centric transcriptome indexing. I'll consider how this might be implemented.