tf 0.4.0

New features

• tf_register() & co implement registration methods (SRVF, affine, landmark,
  continuous-criterion) for aligning functions and analyzing phase variability.
  Returns a new tf_registration class with accessors tf_aligned(),
  tf_inv_warps(), tf_template().
• tf_depth() now implements five functional depth methods: MBD (default), MHI,
  FM (Fraiman-Muniz), FSD (Functional Spatial Depth), and RPD (Regularized
  Projection Depth). MHI ranks functions from lowest (0) to highest (1); all
  other depths are scaled from 0 (most extreme) to 1 (most central).
  MBD is now scaled to [0, 1] instead of [0, 0.5].
• tf_split() / tf_combine() for splitting functions into sub-domain
  fragments and re-combining them
• tf_rgp() gains a "brownian_bridge" covariance option and can generate
  irregular data via the new irreg argument
• tfb_spline() now converts fda::fd and fda::fdSmooth objects directly
• mgcv-style Fourier basis constructor for use in tfb_spline()
• sparklines in print() / format() for all tf subtypes
• tf_where() now defaults to arg = tf_arg(x), consistent with other functions
• as.matrix() for irregular tfd now always interpolates to a common grid

Bug fixes

• tf_derive() for tfd objects now uses second-order accurate finite
  differences, preserving the original domain and grid (#59)
• major overhaul of NA handling: NA entries are now consistently represented as
  NULL internally; fixes for tf_arg(), tf_evaluate(), tf_smooth(),
  tfd() conversion, is.na(), printing, and arithmetic with NA entries
• fix summary.tf() returning wrong central range
• fix tfd domain enforcement for length-1 data (#139)
• fix printing for empty tfb objects and all-NA vectors

Other changes

• tf_derive and tf_integrate can now handle tfb with non-identity link functions
  by falling back to tfd-calculus methods (and returning tfd objects).
• tfd_irreg arithmetic now operates on intersection of arg-values instead
  of failing when arg-values are not identical
• tfb arithmetic fix
• dropped glue dependency in favour of cli / sprintf;
  replaced stopifnot() with informative cli::cli_abort() throughout
• use vctrs::vec_arith() for group generics, vctrs utils in [.tf
• various internal performance improvements
• use air formatter for all R code