Parameterize fixtures (multiple CRS, size, etc.)

[spjuhel]

Changes by @peanutfun:

I think we need to parameterize asap: Use multiple CRS, build exposures of size 1 and >1, etc.

From:

Originally posted by @peanutfun in #1273 (comment)

[peanutfun]

@chahank We should discuss this as soon as possible