Support multi-dim. resample

[dkirkby]

The resample method currently requires that its input be 1D, i.e., a single spectrum. Can this be generalized to to support resampling of multiple spectra with a single operation? There are two use-cases that need to work:

• Each spectrum has its own input wavelength grid, defined by a named input array field.
• All spectra share the same input wavelength grid, specified by an external array.