update docs for WPS process chaining

[cehbrecht]

Description

Recently there were questions about WPS process chaining in the pywps-dev mailing list. Would be nice if we could have some documentation, examples and tests for this. Probably there might also some bugs we need to fix.

Environment

• operating system:
• Python version:
• PyWPS version: 4.2.1
• source/distribution
• git clone
• Debian
• PyPI
• zip/tar.gz
• other (please specify):
• web server
• Apache/mod_wsgi
• CGI
• other (please specify):

Steps to Reproduce

Additional Information

See pywps-mailing list:

• https://lists.osgeo.org/pipermail/pywps-dev/2018-December/001366.html
• https://lists.osgeo.org/pipermail/pywps-dev/2018-November/001345.html