Are we Sphexish?

A round-out release, a few functions and features that I need often.

• new functions xlim and ylim, supporting anything understandable by spex

• new extent method for sf objects, otherwise missing in the core packages

• new spex support for sf objects

• spex() with no input argument will return the extents of the current par 'usr' setting (if only graphics devices had metadata registration for the space in use ...)

• polygonize, qm_rasterToPolygons, and qm_rasterToPolygons_sp gain na.rm
  argument to optionally remove any cells that are NA (in the first layer)

The na.rm need came directly out of an exploratory project needing to treat regions within images as separate tiles for a systematic survey, searching for seals in the ice pack.

first release to CRAN

spex, get a SpatialPolygonsDataFrame extent