first try with regexp for arguments
Added documentation on use of regex

Here you can use regex to refer to argument in a list
For example in
t1: TORSION ATOMS=5,7,9,15
t2: TORSION ATOMS=7,9,15,17
d1: DISTANCE ATOMS=7,17 COMPONENTS
PRINT ARG=(d1.[xy]|t[0-9]) STRIDE=100 FILE=colvar FMT=%8.4f
Slight optimization in rational switching functions

* Better algorithm for int powers
* Simplification when 2*N=M (which is the usual case)
Modified the way groups are read in in multicolvar

You can now read in groups with names other than GROUPA, GROUPB and
GROUPC.  Also output of the atoms in groups is neater.  Lastly I realized that I
had accidentally added some crap to regtest/multicovar/rt45 - I have
now deleted this.
Added BRIDGE colvar, which is equivalent to PLUMED1 waterbridge
Replica echange + neighbor lists do not work

I added a check that disables this combination which can
be problematic. I had to also update a regtest
Added regtest. Compile with -D__PLUMED_HAS_CREGEX
Clarified usage of dot in regexp
regtest consistency
Merge branch 'args-with-regex' into v2.0
Fixed regest (directives were erroneously commented)
forgot files in last commit sorry
Merge branch 'v2.0' into v2.0-hrex