sps 0.5.4

• Added becomes_ta() to determine the sample size when a unit enter the
  take-all stratum.

• Internal changes to the way classes are instantiated. No user-visible changes.

• Updated maintainer email.

sps 0.5.3

• Documentation only; no changes to any functions.

Version 0.5.2

• Added a vignette.

• A single sample size is now recycled for each stratum when drawing samples or calculating inclusion probabilities.

• Added a cutoff argument that puts units with sizes above the cutoff into a take-all stratum.

• sps objects now inherit from numeric instead of integer.

• Replacement methods for length and levels no longer mangle sps objects.