Adds error checking for `--smear_method` option.
Adds error checking for `--smear_method` option.

Uses `parser.error` method to raise an error if `--smear_method` specified is
not recognised as a valid smear method. Added a note to documentation.

Also responds to other comments:
  * Two lines slightly longer rather than new line, preferred for readability
  * pep8 checked, no errors apart from the two lines that go over 79 characters

EdLeming/echidna,jwaterfield/echidna,drjeannewilson/echidna,snoplusuk/echidna,mjmottram/echidna,ashleyrback/echidna,arushanova/echidna