Removed regtest analysis/rt43

This regtest always fails because it was not very well designed.
Furthermore, the functionality being tested by this test is checked in analysis/rt0
so keeping this bad test seems not worthwhile.
Removed tests of multicolvar neighbor list

Multicolvar neighbor lists are being removed and replaced by link
cells.  As such the tests on these quantities are no longer needed
so I have removed them.
Using link cells in many more multicolvars

I have removed all the old and complicated neighbor list stuff
from multicolvars and replaced this with link cells.  These are
used whenever you have switching functions.
Removed dynamic all_atoms from multicolvar

In multicolvar the list of atoms used to update when the
neighbor list changed.  I think this did very little for
the speed of the code.  What it certainly did was make
editing confusing.  I have thus removed this feature and
made it so that the list of atom indexes is hard set during
construction and is fixed for the duration of the run.
Using link cells in DISTANCES