feat(GridIntersect): #902 (#903)

Add gridintersect to flopy utils along with gridintersect tests.

Add keepzerolengths option to strtree method

Add gridintersection demo notebook

Update travis to install shapely

New method for parsing shapely intersection results
fixes failing test caused by change in behavior in GEOS and shapely
clean up and improve code/docstrings

Ad triangle grid for vertex grid tests

Update notebook with triangle grid

Fix conversion of cell2d to list of shapely shapes
using ncvert to determine no. of points

Add option to perform intersections without building STRtree

Add filter to STRtree query results to avoid unnecessary intersection calls

closes #902

Fix transform of MultiPolygon coords

Add support for transforming MultiPolygon coordinates into
real-world coordinates in structured mode.