NoData handling is not ideal for condition1, condition2, condition1_future, and condition2_future

[vlandau]

Currently, Circuitscape.read_raster overwrites NoData in the rasters with -9999. In (possibly rare) cases, the range of possible values for the condition rasters being fed into Omniscape could include -9999, so setting NoData pixels to -9999 in the array representation would mess things up.

I'm not sure the best way to handle this. One less-than-ideal option comes to mind:

• Impose restrictions on the range of acceptable values for the condition layers so that -9999 is far enough away from the minimum value that it won't interfere with source-target matching.

For now, I'll update the docs to reflect this issue, but I will leave this open until I arrive at a better solution.

EDIT: Duh :facepalm: I'll using missing.

[vlandau]

Closing and dealing with this on #69