Allows a full set of data to be downloaded once, and then subseuqnt
analysis to simply cut from that file. Needs to be better intergrated
into the whole system.

implement prior generation

See merge request Monash/peyote!23

# Conflicts:
#   peyote/prior.py
#   tutorials/BasicTutorial.py
#   tutorials/Injection.py

Spin phase angles

See merge request Monash/peyote!26