HDF5 Updates/Improvements, 1.0 branch (2017.11.23.)

I spent a bit of time on cleaning up @dguest's code.

The updates in the build configuration of the package are very minor, but I wanted to get it in sync with all of the other packages that we have.

After some deliberation I decided to move FindHDF5.cmake into the AtlasLCG package, to make it possible to use LCG's HDF5 installation in projects that don't build their own version of this external. Since I did this, I updated the module to make use of the helper code in LCGFunctions.cmake.

Finally, I spent a bit of time making the build work correctly as part of AthAnalysisExternals as well. To be able to provide the exact same version of HDF5 to both AnalysisBase and AthAnalysis users. (This was not all that trivial, as CMake's own FindHDF5.cmake module kept being very confused by where it should pick up stuff from, the LCG release, or our own installation.)

Since this modifies the externals built by AthAnalysisExternals, pinging @will as well.