simpeg/discretize: serialization of meshes

Update instantiation of basemesh so meshes can be formed from their serialized state, e.g.

 mesh = discretize.BaseMesh.deserialize(mesh_json)

will have the value of _n stored, thus, n does not need to be provided on instantiation

Update the x0 property on the TreeMesh so that it goes through the

Allow complex scalars to return true in the isScalar util, as well as numpy.number (rather than specifically np.int, np.float)

Add testing on the serialization / deserialization of meshes