Added documentation on --enable-rpath

It now takes paths into account correctly

This makes it possible to use the command

port test plumed

to compile and test plumed.

Notice that tests on travis are still performed explicitly,
I plan to fix this in the near future, after having verified that there
are no problems with timings.

Minor changes to VES manual

cc @valsson

Using ...Args it is easy to forward any argument to the constructor.

This allows avoiding to explicitly put "new" statements when initializing
a ForwardDecl. In addition, all ForwardDecl that should be constructed
without arguments can be omitted from the initialization list, which
significantly simplifies the code.

See https://groups.google.com/d/msgid/plumed-users/8fcca53b-38d1-470e-a413-a4339702c6db%40googlegroups.com

Thanks to Qingh Liao

Change intel pragmas to OpenMP v4+ pragmas

cc @HanatoK I fixed the problem. Namely, plumed was linking by mistake
another boost version (homebrew).

HomeBrew boost was linked by mistake.

changed some interfaces to return a unique_ptr