Clean up package build NOTEs

[roninsightrx]

NOTEs below should be avoided.

❯ checking top-level files ... NOTE
  Non-standard file/directory found at top level:
    ‘docs’

❯ checking dependencies in R code ... NOTE
  Unexported object imported by a ':::' call: ‘ggplot2:::label_variable’
    See the note in ?`:::` about the use of this operator.

❯ checking R code for possible problems ... NOTE
  Found if() conditions comparing class() to string:
  File ‘vpc/R/add_stratification.R’: if (class(dat$strat) != "factor") ...
  File ‘vpc/R/binning.R’: if (class(n_bins) != "numeric" | is.null(n_bins)) ...
  File ‘vpc/R/define_bins.R’: if (class(bins) != "numeric") ...
  File ‘vpc/R/define_bins.R’: if (!is.null(kmmc) & (class(bins) == "logical" && bins == FALSE)) ...
  File ‘vpc/R/define_bins.R’: if (!(class(bins) == "logical" && bins == FALSE)) ...
  File ‘vpc/R/define_bins.R’: if (class(bins) == "logical" && bins == TRUE) ...
  File ‘vpc/R/define_bins.R’: if (class(bins) == "character") ...
  File ‘vpc/R/define_bins.R’: if (class(bins) == "numeric") ...
  File ‘vpc/R/plot_vpc.R’: if (is.null(vpc_theme) || (class(vpc_theme) != "vpc_theme")) ...
  File ‘vpc/R/plot_vpc.R’: if (!(class(bins) == "logical" && bins == FALSE)) ...
  Use inherits() (or maybe is()) instead.