{Readme; Tests; MC state variables} update

Hi Sacha @sacha.duverger, could you please look at the following questions (if not the whole MR, depending on available time)

1. you quote both but it is not clear for me whether just make or make install are necessary (in connection with below point ?)

2. I still can not use -DINSTALL_DIR (and probably never could). Always get a startup error about a missing libjawt.so (from package openjdk-11-jdk, where would this come from by the way ?...) if I try to. Can you ? If not, I can finally get used of build/mpm, but let us avoid advertising about -DINSTALL_DIR then ? If yes, are executable copied (as currently written in README) or moved ?

3. do you confirm both cmake options MPM_BUILD_TESTING (before !19 (merged), in public version) and WITH_TESTS are equivalent ? (was there a particular reason for replacing one with another ?)

4. an idea whether we do still need libhdf5-serial-dev as a dependency ? And what about libxt-dev ?