Allow user to manually specify EPSG for output projection

[scottstanie]

Currently are getting the most common projection from the input SLCs here https://github.com/isce-framework/dolphin/blob/ff6e4b0f878e271c92e285947b72b681249115ab/src/dolphin/stitching.py#L186 then using that to warp any difference inputs to the same projection before stitching here: https://github.com/isce-framework/dolphin/blob/ff6e4b0f878e271c92e285947b72b681249115ab/src/dolphin/stitching.py#L353-L357

This is fine for most cases, but we may want to specify one EPSG for all stitching calls. If we have a frame which falls on the border of two UTM zones, and there's a chance that a frame with missing data for some dates will get stitched to a different EPSG than the other images.

Additionally, we should do another check before running gdal_merge that the bounds we are passing make sense with the projection, as right now we are assuming that the bounds match the most-common projection