Continent contour buffers on individual continent polygons

[jcannon-gplates]

Each continent polygon can now have its own buffer expansion (in ocean-ward direction).

This functionality addresses the problem of shrinking of continent contours for recent geological times: A typical use case had buffering for pre-pangea times (to account for larger continents before collisions reduced them) and gradually reduced buffering to zero for post-pangea times. However the shrinking was quite obvious. To make it less obvious, manually-drawn COBs (for the large continents) were then used (which don't need buffering). But don't have these for small terranes. With the new functionality, the small terranes can be identified by their plate ID and given a non-zero buffer.

• Each reconstructed continent polygon gets its own buffer distance (eg, can be based on its plate ID).
• Can only specify buffers for continent contours or buffers for continent polygons, not both.
• If specifying buffers for continent contours (instead of polygons) then can now also access the reconstructed continent polygons contained within each contour (though not as useful as buffering each polygon individually).
• Individual polygon buffering enables continent contours to be nice and stable, and not change when smaller contours join into larger contours over time (as was the case with specifying buffers for continent contours).