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.

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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

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

This reverts commit dcd4a63d.

This reverts commit 3758d552.

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this is usually not important, but for the case (like with
metainference) in which you have a bias that is applied on
a large number of arguments, then it was getting very slow

This reverts s hunk in commit 417f97b8
that triggers a bug in my compiler

I cleaned up some of the (innocuous) warnings

I think there was a typo here @gtribello

the compiler of my mac says this call to pow is ambiguous

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The new message class (information) is treated as (style).

I temporarily add two travis test to allow 1.71 to fail.

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I modified extensively the travis file to allow multi-os support
and to use more recent UBUNTU:
- I upgraded most of the tests to UBUNTU trusty (with gcc 4.8)
- For a couple of tests I switch to UBUNTU precise (with gcc 4.6)
  to be sure we keep compatibility with older compilers
- I removed clang tests from UBUNTU and added OSX tests (both serial and MPI)

Since UBUNTU trusty comes with doxygen 1.8.6, I install doxygen 1.8.7
from source. Notice that this was solved in UBUNTU precise by using
an external ppa database. I prefer install from source here
since in this way we can easily upgrade doxygen to use latest features.

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