ENH: speed up image driver startup time

[jhamman]

@yixinmao and I have noticed that the initialization of the image driver is progressing quite slowly. This is likely due to the way we are reading the parameter files (one variable at a time with multiple opens/closes of the file). A minor refactor will speed this up.

This is likely related to #380

[bartnijssen]

@yixinmao : Since this is affecting your run-time in your data assimilation runs, I am making you the point person on this one. I mostly refer you to #380, since fixing that (keep netcdf files open) is likely to resolve much of the delay.