Use Class Methods for Grid Construction (i.e. Grid.from_verticies())

[philipc2]

Originally commented by @dcherian in #19

Here are a few suggestions

uxarray.Grid.init(self, string file)
(*) uxarray.Grid.init(self, string gridspec)
uxarray.Grid.init(self, np.float64.list vertices)
How about making the last two classmethods instead to avoid overloading init

grid = uxarray.Grid.from_gridspec(...),
grid = uxarray.Grid.from_vertices(...)
Even Grid.from_file looks nice to me but if that's the most common usecase, perhaps Grid.init(self, file) would be most convenient.