Joint-RPCA improvement

[antagomir]

Calling df_mia <- getJointRPCA(...) is returning two attributes among others:

attr(df_mia, "varExplained")  # explained variance 
attr(df_mia, "percentVar")     # explained proportion of variance i.e. x^2 / sum(x^2) where x is explained variance

The naming is potentially confusing. One might consider that "varExplained" is already the explained percentage.

Suggestion: rename percentVar into propVarExplained

[antagomir]

@sabujcb and @RiboRings can you have a look at this together, and we can then discuss when anything needs to be clarified.

[RiboRings]

If I remember correctly, percentVar and varExplained are standard names in the results of many classic R functions for dimred (or actually in Python, I don't remember). I would:

1. clarify their respective content in the vignettes
2. Check what names are used in the results of similar mia/scater functions for dimred