remove reliance on reference_sce

[moi-taiga]

The functions:

acccuracy_test()
reference_idents()
ppr_vioplot()

Currently require reference_sce.
When PPR is shipped with pre-computed trajectories reference_sce will not be available.
Therefore the functions should be set up to accept whatever alternative to reference_sce is used.
perhaps the metadata only, which includes cell names identities and pseudo-times.

precision will still require reference_sce().

[moi-taiga]

acccuracy_test() will still require refernece_sce, it should in fact only require refernce_sce; as per issue #45 .