snaputils: SNAP-specific utilities for Shiny apps.

The snaputils package contains utilities for Shiny app development. The package inherits primarily from `apputils`, which contains common utility functions, settings and references for use across multiple Shiny apps. It also imports maputils. As the name suggests, snaputils is specific to a SNAP context. So is maaputils though separate. Only apputils has a more general non-SNAP context. All three packages are satellite members of the SNAPverse collection of R packages.