update version to 6.31.4 as pushed version to Bioconductor

Merge pull request #325 from mixOmicsTeam/update-readme

Update readme

PR addressing Issue #214 (#218)

fix: implemented BiocParallel functionality into `tune.spls()` via restructuring of the main portion of the function. bplapply() iterates over all pairs of input keepX/Y so only reduces runtime for long lists of test.keepX/Y

refactor: previous PR left duplicate function used for development. this is rectified and more comments are added

test: added test cases for `tune.spls()`. These are just placeholders as more robust testing criteria needs to be established as well as this document will be fully updated as part of PR #206

tests: updated tests to assess actual values alongside RNG control of parallel function calls.

refactor: replaced all calls to `cpus` to `BPPARAM` in `tune()`

docs: updated `tune.Rd` file with cpus -> BPPARAM shift