Previously, when the first xml document was finished and the XMLDocument
object was deleted, then xerces would be uninitialized. This lead to
the parser being invalid so the next read of LHEReader would crash.
Keeping the XercesPlatform alive for the lifetime of the LHEReader
fixes that problem.

Moved to std::unique_ptr and std::shared_ptr for most memory handling.
Changed member variables to match CMS conventions.

When we identify that we need to merge the LHERunInfoProduct in
nextEvent, we immediately do the merge.
This avoids the need for a endRun call which will allow those calls
to be removed from the framework.

The function fileIndex was not being overridden for base class
ProducerSourceBase. Now we forward FromFile::fileIndex to
ProducerSourceBase.

By telling the framework that the LHESource has gone to a new file,
the merging of RunInfoProduct can be done properly.