* Citation fixed as suggested
* Added PBC-friendly virial contributions: compared with the numerical derivatives
  for a configuration and the same with a dimer shifted by the lattice vector on one of its components.

the regtext might produce different results on other architectures so I added
a numeric format keyword to the plumed.dat file of both regtexts.
Also the test .pdb configurations are improved, being now taken from a trajectory rather than
a starting configuration.

Added other failing examples for the regtest based on exceptions.

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In case of error with LOAD, the code was returning an error without
raising an exception.

This should make the code slightly fast.
Notice that it is now necessary to explicitly sort the watches
when printing the log.

These files were related to the original implementation of RMSD.
I think they have not been used for years, so I remove them.

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This is done only in the cmd interpreter

This should make the code slightly fast.

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Now all installers use a temporary directory

TODO:
- Tidy up the input
- Regtext
- A more extensive documentation

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Notice that even though we assume c++11 to be available
in PLUMED 2.4 I here protected the definitions so that
they are only available with a c++11-supporting compilers
(see comments in the code).

This should not be necessary in other parts of the code,
since autoconf is already checking that c++11 is supported.

This requires a few fixes:
- Replace -ansi with -std=c++11 in autoconf.
- Add a check that c++11 is supported in autoconf.
- Change flags for cppcheck.
- Remove travis configurations that would fail.

Additionally, I removed the check for 'explicit' keyword,
which is not needed with C++11.

Fixes #212

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Actually this should have been done together with 21be029e

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