Allow user to specify output spacing in meters, instead of just `strides`

[scottstanie]

Right now there is a field for output_resolution, but it's not implemented: the strides field must be used to downsample the output.

This should be a relatively simple feature to

• get the pixel size/ GeoTransform of the input data
• convert the meters given for the output_resolution into a strides
• overwrite the strides field (raising an error if they tried to specify both strides and resolution)

Right now we're not attempted to allow a different output projection than the input SLCs, so we shouldn't need to account for any differences in units.