Add NUMMEMBERS variable?

Hi autosubmit team,

As a suggestion, in some cases it may be interesting to have a NUMMEMBERS variable, like NUMCHUNKS (http://autosubmit.readthedocs.io/en/latest/variables.html#project-variables), totalling the number of members defined in the project (in MEMBERS)

It could be useful for early detection of experiment miss-configurations made by an user, when an experiment runs an ensemble (>1 member) and some other variables needed for the ensemble are not defined in a .conf file

If you consider that another alternative for auto-models could be better I'd grateful to use it :)

Thanks!