Renamed main script to `expression-simulation.sh` and created a new script
that stops after the transcriptome assembly called `transcriptome-simulation.sh`.

Modified `sample-fasta.R` to output mean length of transcripts. Could ultimately
use this to determine number of reads generated by simNGS to a set coverage level.
New known transcript file that is randomly selected from first 1000 downloaded to avoid closely related sequences in original file that was simply the first 100 downloaded