Check for valid amino acid sequences in the input

[guzmanfj]

I got this error when there was an X in a protein sequence

  File "/Users/guzmanfj/Documents/py/CoolerCodonOpt/scripts/codonopt.py", line 482, in opt_backtranslate
    codons = codons_probabilities[aa][0]
KeyError: 'X'

Input sequence:

>118
MFVEVPPENAIXTYYATSVLIIQIQDFYTPLNPANMLTLQQVIREPRDIRLTPRLDVQAQ
PKVWPS