Adding shapely as an optional dependancy

[briochh]

@rhugman-intera has some methods for locating pilot points on grid, leaning on shapely: #387 (comment) . I don't see any obvious reason we can have shapely as an optional dependancy (we might already be some other methods using it?). Thoughts @jtwhite79?

[jtwhite79]

Can we isolate that dependency so that it's only imported if someone calls that function?

[rhugman]

Sure - imports can be called at the function level?

[jtwhite79]

I think so - so we just try-catch on the import shapely within the fxn that needs it. That would let those who dont need that dep continue without it...I think FloPy used to do this also (or maybe still does in a few places?)