Implement 2-D spectra

To replace internal implementations of Mantid-Abins we need to also support instruments such as MARI. This shouldn't require much change to the current internals of abinslib and is a natural follow-up to #3 .

*Duplicated from https://github.com/pace-neutrons/abinslib/issues/36*

Start by reviewing the current abinslib internals and the related issue #3, then compare them with the duplicated issue at pace-neutrons/abinslib#36. Define the work needed to support 2-D spectra for instruments such as MARI and to replace the corresponding Mantid-Abins implementations; done means the new instrument use case is supported without unnecessary internal changes.

Written by the indexing model from the issue text.