merge_rasters cannot handle mixed 3/4 band rasters

[mikemahoney218]

The NAIP endpoint has a tendency, when transparent = TRUE, to return a mix of 3 and 4 band images (I imagine 3 when there are no transparent pixels and 4 when there are, but I haven't validated this). This causes problems with at least my naive use of gdalwarp, where the output raster is assumed to have the same number of bands as the first input file. The older implementation of merge_rasters is much slower in this case (by virtue of reading every file into memory) but doesn't error out.

In the short term, I plan to fix this by reintroducing the older code as a fallback mechanism. In the mid term, I think I can handle this particular case by the -dstnodata flag, and make it less common by changing the default transparency argument for the NAIP endpoint.

[mikemahoney218]

Short-term fix implemented as #32, closing