Add optional extra params to processes

[cimendes]

An option to add extra params that we currently do not contemplate in the process is necessary as it will allow users to customise the process.

A possible solution is to add this options in the self.params() of each process:

self.params = {
  "krakenDB": {
    "default": "'minikraken_20171013_4GB'",
      "description": "Specifies kraken database."
  },
  "krakenExtra": {
    "default": "",
    "description": "Add any extra params"
  }
}

Note that the name of the param will have to be indicative of which process it's referring to. A way to overcome this is to create a new attribute self.extraParam but it might over complicate things. The solution presented above is simple although a bit laborious.