Add Catchment Join and Split Functionality

This minor release adds catchment polygon functionality to go along with the flowline refactor functionality released in v0.3.0. Some bug fixes and package cleanup was also taken care of. This introduces a dependency on the raster.

The attached nhdplusTools_0.3.1.tar.gz contains package source and vignettes. See the package down site for full documentation.

The catchment functionality can perform the kind of transformation shown in this gif.