Add grouping to gen2 dither scripts

We want to group all exposures in a dither sequence under a common id. From the Gen2 point of view, the mechanism will be as follows.

Before starting the dither loop, request a new group from iic getGroupId groupName=$NAME, where I recommend using "dither" for the $NAME.

The response will be an integer in PFS.GROUPIDS.$NAME.ID. That id should then be passed the exposure commands with groupId=$ID.

For example, in semi-Gen2 pseudo-code:

exec iic getGroupId groupName=dither

ditherGroupId = !PFS.GROUPIDS.DITHER.ID

for ditherPos in ditherPosList {
   exec icc scienceObject exptime=90 window=500,1000 groupId=ditherGroupId
}

If you need to restart a dither sequence (to add a new ring, for example), you can request the groupId for an existing group with iic getGroupId groupName=dither continue. If you do not add that continue you will get a new id.

[ I might add .VISIT or .DATE to the PFS.GROUPIDS.DITHER dictionary, which is why there is an additional .ID ]

This ticket will be blocked