Add LFO-CV to projpred

[yannmclatchie]

This PR brings to projpred:

1. a new ordered search method, where we simply append the next predictor in time; and,
2. one-step-ahead LFO-CV for submodel validation.

[fweber144]

Just for future reference: The following shouldn't be forgotten before merging:

• Unit tests
• NEWS.md entry (also mentioning you and possibly other contributors)
• Perhaps add a new vignette?

[fweber144]

I think we should mention somewhere that within projpred, "LFO CV" actually means "1-SAP-LFO CV" (one-step-ahead prediction [or predictive?] leave-future-out cross-validation). Perhaps in the docs as well as in the NEWS.md entry?