update docs for WPS process chaining

# 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