(cherry picked from commit ae6c8ee4)

This will decrease the number of pull requests from PlumedBot.

Currently, everytime a file is updated and someone forgets to run
headers.sh, the headers are outdated. With this change, this will happen
once per year. PlumedBot will thus mostly fix erroneous copyrights
(rarely happening).

Plumedbot pull request

This commit contains changes automatically generated by plumedbot on Travis-ci.

Plumedbot pull request

This commit contains changes automatically generated by plumedbot on Travis-ci.

Since isnan() is only available in c++11, I prefer not to backport
this so v2.3.

At variance with older commit fba71ad4
I here only perform the check at the first step

Now also crashes with TYPE=SIMPLE and, I guess, with all reference
structures

Only for
RMSD TYPE=OPTIMAL

Fixes #330

Plumed.cmake is now treated as Plumed.inc