When doxygen is not found, doc is disabled

Pdf manual takes a lot of time to be compiled.
Since it is not very useful, I disable it by default.

Notice that on travis CI it is enables during configure.

[makedoc]

[makedoc]

[makedoc]

notice of 30734458

With this change it looks I can solve an issue on my workstation
that has been reported on the mailing list (multiple linking
of Plumed.o object)

some files where not up to date

[makedoc]

- faster calculation for RADIUS and TRACE
- default is now NOT_MASS_WEIGHTED
- regtest updated

Gaussian kernel functions were normalised incorrectly.  This should
only cause a multiplicative scaling of the histograms calculated
using the HISTOGRAM action

Added option in driver to evaluate forces due to a bias numerically so that they can be compared with the analytical forces evaluated by plumed

[makedoc]

- Implementation of short help (with :PHelp)
- Updated doc
- Autocompletion now knows if label was already provided at the beginning
  of the line and does not suggest it anymore in case
- Compilation of the vim plugin is more similar to compilation
  of the user doc
- Compilation of the vim plugin is made when doing "make install"
  from root dir.

This reverts commit dcd4a63d.

This reverts commit 3758d552.