Account for X aminoacid letter

[francosimonetti]

I found this issue, maybe it's a good idea to account for unknown aminoacid letters in case the sequences are not sanitized before.

worker_1_7b57679bcd66 | File "/usr/local/lib/python3.7/site-packages/amphipathic/init.py", line 139, in index
worker_1_7b57679bcd66 | index_secondary(seq)
worker_1_7b57679bcd66 | File "/usr/local/lib/python3.7/site-packages/amphipathic/init.py", line 132, in index_secondary
worker_1_7b57679bcd66 | for protein in seq.resume_secondary()
worker_1_7b57679bcd66 | File "/usr/local/lib/python3.7/site-packages/amphipathic/init.py", line 132, in
worker_1_7b57679bcd66 | for protein in seq.resume_secondary()
worker_1_7b57679bcd66 | File "/usr/local/lib/python3.7/site-packages/amphipathic/init.py", line 130, in
worker_1_7b57679bcd66 | for aa in protein
worker_1_7b57679bcd66 | File "/usr/local/lib/python3.7/site-packages/amphipathic/init.py", line 116, in add_indexes_to_secuence
worker_1_7b57679bcd66 | index, mean = calculate_amphipathic_index(struct)
worker_1_7b57679bcd66 | File "/usr/local/lib/python3.7/site-packages/amphipathic/init.py", line 105, in calculate_amphipathic_index
worker_1_7b57679bcd66 | mean = sum([hydrophobic.table[aa] for aa in seq]) / amount
worker_1_7b57679bcd66 | File "/usr/local/lib/python3.7/site-packages/amphipathic/init.py", line 105, in
worker_1_7b57679bcd66 | mean = sum([hydrophobic.table[aa] for aa in seq]) / amount
worker_1_7b57679bcd66 | KeyError: 'x'