Add cilNexusReader and cilNexusResampleReader

[lauramurgatroyd]

These will be specific cases of the cilHDF5Reader and cilHDF5ResampleReader, that will be setup to read the cil Nexus format.

Note that the cil nexus format may include data, dark fields and flat fields. The readers should be able to read all of these and return each of them separately. How it works will depend on how we adjust the cil nexus format to include dark and flat fields, see TomographicImaging/CIL#934 .

If we want to write the results of resampling with the cilNexusReader to a file, this will need to contain the dark and flat fields, but also the origin and spacing of the image that will need to be used to display it correctly with the viewer if this file was read back in.
We can either write out a cilnexus file and add extra attributes for the origin and spacing - in this case we need to come up with some convention for this. Or we can write out 3 metaimage files (which already have the origin and spacing as settings in the header).