regtest is executed only if plumed --has-matheval

Added more ways to check if tools are installed. Now also
plumed --has-matheval
is allowed

I adapted the debug replica exchange stuff. Since running on multiple
trajectory could be useful, I leave --multi as a non-hidden option.

I slightly changed behavior of the hidden flag --debug-grex (now one sets the exchange stride, not the number of replicas - the latter is set with --multi). Thus, a couple of retests had to be updated

At variance with EXTERNAL,
this bias is used when calculating the WT hills height

Now getReducedList can be called more than once returning the same list
I also updated a regtest.
The previous test was only testing stride=2 (or, equivalently,
stride=20 into a trajectory with stride 10).

- solved a bug with averaging over parallelized replicas
- more documentation

Note: the default bin calculation is reintroduced in sum_hills not to break the test

to reflect the implementatin of INTERVAL and ALPHABETA

and added the doxygen part

also with grid but without splines (automatically deactivated).

Now the stride for the neighbor lists in the path implementation
are in timesteps. By now this means that PATHMSD and PROPERTYMAP are
defined with e.g. NEIGH_STRIDE=5