Improve support for sparse volumes

[trhallam]

xarray had limited sparse volume support at last check, see if this has been improved recently and map out whether it can be added to segysak.

[trhallam]

Sparse volume support won't be supported. Instead, Xarray backend has been added in v0.5 for lazy loading which should allieviate the need for excessively sparse volumes. Alternatively, uses can load SEG-Y data as 2D data (naturally sparse) with dimensions has addtional byte fields. Sub-selections of data can then be stacked to create the volume views necessary by the user.