MCOLV_COMBINE: Improve documentation and allow more than two input multicolvars

[PabloPiaggi]

I noticed that there are no examples in the documentation of MCOLV_COMBINE and it would be nice to add one.

Furthermore it seems that one cannot use more than two multicolvars in the DATA keyword. If I try to use three multicolvars I get the cryptic error:

+++ PLUMED error
+++ at LinkCells.cpp:161, function std::array<unsigned int, 3> PLMD::LinkCells::findMyCell(const Vector&) const
+++ assertion failed: celn[j]>=0 && celn[j]<ncells[j]

It would be useful to have a clear error message or even better to be able to use any number of multicolvars as inputs.

MCOLV_PRODUCT might have similar problems.