Add NcML to supported FORMATS

[huard]

Overview

Adds the NcML format to the list of supported file formats.

Related Issue / Discussion

Fixes #514

Additional Information

Contribution Agreement

(as per https://github.com/geopython/pywps/blob/master/CONTRIBUTING.rst#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to PyWPS. I confirm that my contributions to PyWPS will be compatible with the PyWPS license guidelines at the time of contribution.
• I have already previously agreed to the PyWPS Contributions and Licensing Guidelines

[coveralls]

Coverage remained the same at 75.618% when pulling b4b5801 on bird-house:fix-514 into cbca3cf on geopython:master.

[jachym]

scary

thank you @huard