Don't use boundary grid points for optimization

[JRLoveridge]

Very small inconsistencies in the calculation of the sensitivity of the direct beam optical path to the extinction, typically in boundary voxels. Until this is resolved, I suggest not using these grid points for retrievals.

The errors includes some negative values in the predicted sensitivity. Currently, the tests for the direct beam fail but the accuracy loss is very small (maximum_error < 1e-4) for typical values ranging two orders of magnitude larger. Derivative calculations within the volume should still meet the specified accuracy requirement in the tests.