allow LHEPrimaryGenerator to shift vertex away from the origin

[tomeichlersmith]

Is your feature request related to a problem? Please describe.
Some folks want to have their LHE events start somewhere besides the origin and they do not want to go through the hassle of rewriting their LHE files with our custom #vertex... syntax within each <event> block.

Describe the solution you'd like
Update the configuration to accept a 3- or 4-component position vector as the event vertex.

ldmx-sw/SimCore/python/generators.py

Lines 84 to 98 in d74f8cc

	class lhe(simcfg.PrimaryGenerator) :
	"""New LHE file primary generator
	Parameters
	----------
	name : str
	name of new primary generator
	filePath : str
	path to LHE file containing the primary vertices
	"""
	def __init__(self,name,filePath):
	super().__init__(name,'simcore::generators::LHEPrimaryGenerator')
	self.filePath = filePath

Apply this vertex as the new "origin" for the LHE events, applying the #vertex ... syntax relative to this value.

ldmx-sw/SimCore/src/SimCore/Generators/LHEPrimaryGenerator.cxx

Lines 38 to 40 in d74f8cc

	G4PrimaryVertex* vertex = new G4PrimaryVertex();
	vertex->SetPosition(lheEvent->getVertex()[0], lheEvent->getVertex()[1],
	lheEvent->getVertex()[2]);

While mucking about with the LHE generator, one could also look at resolving #1311 or even #1316 if you want to get really technical.

Describe alternatives you've considered
The main alternative is to develop a pre-processor that adds the #vertex ... into the <event> blocks. This could be done in Python and then included in a config, but I find that error prone and not "clean".

Additional context
Needed for studying ALPs @sophiemiddleton