Fixes #7

Allows to switch off auto backup or to change the maximum number of 
backups

When MIN keyword was not used, an array was set as empty and later
accessed.

Removed explicit open mode, so that auto backup/auto restart is used also
for DUMPPROJECTIONS, DUMPDERIVATIVES and DUMPFORCES. Regtests has been added
with backup files.

I also removed some unnecessary c_str().

(I refer to issue number 2 in github to test if it
gets closed).

Now replica exchange can be used with plumed even if plumed
does not do anything at the exchange step.

Permissions of installed files were not correct. Now after
install all permissions are reset so that "group" and "others"
can read and execute everything

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