2018-04-21 Check RST syntax

It is usually a pain to discover I made an error in a formula while I’m writing documentation. It fails quite long after after the unit tests started. The documentation is generated after the unit test pass. I also use a lot .. runpython:: (see RunPythonDirective) to run pieces of code inside the documentation. It is quite annoying to discover it fails long after. So I creates a unit test which can be used to compile a single page of the documentation : test_doc_page. I modify the page name when I have some doubt or I move to another one.