Incorrect choice of default axes

[DanPorter]

The recent change in v1.3.0 has introduced an error in some scans that chooses the wrong scan axes:

import hdfmap

m = hdfmap.create_nexus_map('/dls/science/groups/das/ExampleData/i16/azimuths/1108750.nxs')

axes_paths, signal_paths = m.nexus_default_paths()

print('axes_paths:', axes_paths)
print('signal_paths:', signal_paths)

axes_names, signal_names = m.nexus_default_names()

print('axes_names:', axes_names)
print('signal_names:', signal_names)

Returns

axes_paths: ['/entry/measurement/eta_fly_fly']
signal_paths: ['/entry/measurement/mroi2_sum', '/entry/measurement/count_time', '/entry/measurement/merlin_max_val', '/entry/measurement/merlin_max_x', '/entry/measurement/merlin_max_y', '/entry/measurement/merlin_total']
axes_names: ['eta_fly']
signal_names: ['rc', 'count_time', 'merlin_max_val', 'merlin_max_x', 'merlin_max_y', 'merlin_total']

The default signal_name should be mroi2_sum, but this is missing from the signal_names list.

[DanPorter]

This happens as a result of commit: adaefc5

Because duplicates are removed from the list, the order changes and the set of last items becomes different. I need to find a better way to do this.