LPD: combine gain maps

The LPD corrections have the following multiplicative constants: GainAmpMap, RelativeGain, and FFMap. Last I discussed with Hazem, one of them is always just ones. Regardless of what they are, we should just multiply the maps together when loading constants and then, during correction, do a single multiplication instead of up to three (saving loads of memory, too). I see that lpdalgs.pyx in pycalibration just assumes one such ccv_offset input already.

Haven't done this yet because:

1. Maintaining a correct buffer would be tricky (keep track and rebuild when constants are loaded / settings are changed)
2. If the selected corrections differ between preview and full output, we may still need two different buffers - and selecting which to use for which case is not currently trivial

The changed coming in !90 (merged) make 1. easier to address. Not sure about 2., but it's something to think about.

assigned to @hammerd