Changes BGS reobservation scheme to match BGS spec
Changes BGS reobservation scheme to match BGS spec

Changes to calc_numobs in targets.py to implement the following:

- BGS targets can be reobserved on every epoch, there is no maximum
  number of reobservations (although priorities change on ZGOOD).

- BGS targets should be observed no more than once on a single epoch
  (which can happen in tile overlaps).

desihub/desitarget,desihub/desitarget