Improve cluster sources bookkeeping

[gavinevans]

Related to https://github.com/metoppv/mo-blue-team/issues/1244

Description
This builds on #2431 and #2435. Only this commit is actually relevant. This PR modifies the bookkeeping of the cluster_sources attribute within the RealizationClusterAndMatch plugin, so that a given forecast_period only appears in one forecast source, rather than below where 3600 appears in multiple forecast sources. This can cause an issue later, within the temporal interpolation, if there are more than two forecast sources, as it's not clear what we should be temporally interpolating between.

{'ecgl_ens': [np.int32(691200), np.int32(734400), np.int32(777600), np.int32(820800), np.int32(864000), np.int32(907200), np.int32(950400), np.int32(993600), np.int32(1036800), np.int32(1080000), np.int32(1123200), np.int32(1166400), np.int32(1209600), np.int32(1252800)], 'gl_ens': [475200, 518400, 561600, 604800, 648000], 'uk_ens': [3600, 21600, 43200, 86400, 129600, 172800, 216000, 259200, 302400, 345600, 388800, 432000], 'uk_det': [3600, 21600], 'nc_det uk_det': [3600]}

Testing:

• Ran tests and they passed OK
• Added new tests for the new feature(s)