python module should always be called `plumed`, irrespectively of
prefix/suffix. Notice that if installing multiple plumed versions
side by side different python modules will anyway end up in different
directories (`lib/program_name`).

- tested for bias-exchange
- now new force-flags in gromacs so HREX should work equally to 2018
- ENERGY collective varialbe should be tested..

the new organisatoion of the gromacs source code allows simplifying
our patch. Multi-sim is now tested at least for multiple walkers
metad

on a single aladp metadynamics (that works)
more test are needed
- parallel stuff
- replica exchange
- hrex
- gpu

this is not really working yet, but hopefully we are not far

Function was very slow when variables were not present or had zero derivative

Especially when printing size_t it is better to use << operator
which always uses the right size.

`except:` might catch the wrong exception by mistake

this allows parsing the plumed input file by setting the number of atoms without the need of reading a trajectory file
DRIVER does plumed initialisation and then stops

@all this is meant for the plumed-nest, please have a look and comment

@chi2, @chi3, @chi4 and @chi5