not so small-ish fixes

[jeremyrcoyle]

Some small-ish fixes throughout:

• Correct the bounding code in predict to work for a matrix of predictions (from multiple lambda)
• Change the default lambda.min.ratio to 1e-4 (The previous default overpenalized)
• Porting over the changes to the HAL formula functionality originally contributed in Redo formula version of fit_hal #80
• Removal of outdated cpp files that contained documentation but no actual code at all
• Changes to Rcpp documentation to suppress Roxygen errors (NOTE: the line before // [[Rcpp::export]] cannot be a comment (i.e., //), but must be valid Rcpp-Roxygen (e.g., //')
• Reduce redundant Rcpp functions, eliminating get_pnz and get_xscale, which were exact copies of calc_pnz and calc_xscale

[jeremyrcoyle]

@nhejazi, we fixed this up this morning. Should be good to go, pending a look from you. Thanks for your help with it!

[nhejazi]

Great, I just added some more documentation/style edits. Assuming my most recent commits don't fail the CI build checks, then this LGTM for merging.