Add bunch structures/pileup for MC21 testing

Adds

• BunchStructure_Fill7314_BCMSPattern_Flat.py : uniform bunch luminosity (0 or 1 intensity for all bunches)
• BunchStructure_Fill7314_BCMSPattern_Early.py : bunch-by-bunch pattern for LB 200 in fill 7314 (early, less dispersion)
• PileUpProfile_run410000_MC21a_MultiBeamspot.py : PU distribution intended to be used with uniform bunch intensity structure (so it is the convolution of the bunch-to-bunch structure with the average pileup distribution). Uses "step" mechanism to separate the four LB ranges for different beamspot sizes/pileup. Steps 1, 2, 3 are convolution of BunchStructure_Fill7314_BCMSPattern_Early.py with average pileup 52, step 4 is convolution of BunchStructure_Fill7314_BCMSPattern.py (high dispersion) with flat 25-52 average pileup profile.

Attn @jchapman @tlari @tadej ej @demers @jferrand