Multiple energy window code v2

[KrisThielemans]

This is a copy of #722, where we will gradually merge updates to more recent versions of STIR.

I anticipate that when this is done, we will squash history, and generate a new PR.

Expect squashes and rebases therefore.

Pinging @rmapree @ludovicabrusaferri

[KrisThielemans]

I've currently merged rel_5.0.0. (@ludovicabrusaferri @rmapree the conflicts we looked at were related to the #722 was slightly more up-to-date than rel_4.1.0).

This compiles and ctests work. I didn't check recon_test_pack.

Unfortunately, merging git merge rel_5.1.0 gives some more conflicts

	both modified:   src/IO/InputStreamFromROOTFile.cxx
	both modified:   src/include/stir/ExamInfo.h
	both modified:   src/include/stir/IO/InputStreamFromROOTFile.h
	both modified:   src/include/stir/listmode/CListRecordSAFIR.h
	both modified:   src/listmode_buildblock/LmToProjData.cxx
	both modified:   src/scatter_buildblock/ScatterSimulation.cxx
	both modified:   src/scatter_buildblock/scatter_estimate_for_one_scatter_point.cxx
	both modified:   src/scatter_buildblock/upsample_and_fit_scatter_estimate.cxx
	both modified:   src/swig/stir.i

They aren't too bad, but need some attention. I guess this means that more than likely we'll have more of this coming up for later versions.