Support multi-ramp PFSB files

Partly to support visit recovery, and partly to support limiting persistence accumulation by resetting during a NIR exposure, the hxActor needs to fully support exposures (a.k.a. PFS visits) with multiple ramps. The hxhal library itself does support "nramp>1", but the actor does not expose that.

While there I will also make sure that ndrops>0 and ngroups>1 also work. It is far from inconceivable that we will want to be able to save only every Nth read up the ramp, although I'd be inclined to do that at the actor not the ASIC level.

For anyone who might be interested, an ASIC exposure is:

nramp>=1 of
   - nreset>=0
   - group>=1 of
        - nread>=0
        - ndrop>=0

PFS now always runs with nramp=1, nreset=1, ngroup=1, ndrop=0, nread=N