Add snp-sites tool

[pvanheus]

Add a tool for snp-sites

[pvanheus]

Question: output_options are meant to be exclusive. When I create an input object like:

sequences:
  class: File
  path: /home/pvh/Data/fasta/full_msa.fasta
  format: edam:format_1929
output_options:
  output_fasta: true
  output_phylip: true

$namespaces:
  edam: http://edamontology.org/

$schemas:
- http://edamontology.org/EDAM_1.18.owl

A warning results:

snp-sites/tests/snp-sites-test1.yml:7:3: Warning: invalid field `output_phylip`, expected one of:
                                         'output_fasta'

but I would expect execution to not continue, so comments on this would be appreciated.