Remove use of mutable as default value in Process.__init__

[gschwind]

Using mutable value as default value for input variable of function create complex bug because those value will be shared by all Process instance. Thus remove it and replace it by the safe code.

Overview

Related Issue / Discussion

Additional Information

Contribution Agreement

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

• [x] 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.
• [x] I have already previously agreed to the PyWPS Contributions and Licensing Guidelines

[coveralls]

Coverage remained the same at 81.378% when pulling 42dfd67c879ec58b4f8877b5a2e0f1096d3fd17e on gschwind:fix-process-init into 7839f5276ced486a8b7f83379e919fb05273b4e7 on geopython:main.