[Clas12_software] gemc generated events bank

[Maurizio Ungaro]

I'm about to submit some changes to the structure of the generated 
events bank, to make it consistent with the other evio banks.

before:
bank contains pid, px, py, pz, vx, vy, vz
n. banks = n. particles

after:
each quantity pid, px, py, pz, vx, vy, vz  is a bank
n. hits in each bank = n. particles

gemc_evio2root will be updated to read the new format. It won't be able 
to read old files any more.
The evio xml dictionary will be updated as well.

Unless this is an inconvenience for someone, I'll make the change and 
provide the updates this weekend.

best regards,
mauri