Fix harmless bug with combination of group and twin-range

Fix harmless bug with combination of group and twin-range

The combination of
* group cut-off scheme,
* coulomb-modifier != none,
* rcoulomb > rlist (ie. twin-range), and
* rlistlong == -1

was being treated by the wrong clause in readir.c because the check
for "might be zero at cutoff" did not consider the modifier. The only
behavioural difference was issuing a warning that there will be no
buffering.

The modifier checks have been refactored to make it possible to check
the modifier.

Also removed some useless checks for EVDW_SWITCH

Change-Id: Ic64f3f48be17f6d87230bc4060a31e1fb4343973