IMP now requires C++11, so we can now use the
'override' keyword and range-based for loops.

PMI's ProtocolOutput class dropped support for PMI1,
so update to match the new API.

IMP.rmf already saves the score for each restraint,
so we don't need to add it explicitly.

Replace use of std::lgamma with boost::math::lgamma.
The former needs C++11.

When building outside of the main IMP source tree,
we need to find an existing IMP installation. The
FindIMP.cmake script helps CMake to do this.

If the PMI System object has ProtocolObject(s) attached,
notify them whenever we create a new EM restraint, and
provide a dataset pointing to the GMM file we used.