Best practices for WPS

For example:
* handling a set of output files (#13).
* describing more complex inputs (we have “dynamic" inputs in Copernicus with dependencies between the input parameters).

Start by reviewing the output-file example in issue #13 and the described Copernicus use case for dynamic inputs with dependencies. Define the scope of the WPS best-practices guidance, then document recommendations covering both handling sets of output files and describing complex inputs.

Written by the indexing model from the issue text.