serialization of meshes

• from pr: #182
• commits from: @jcapriot, @lheagy
• review from: @jcapriot

summary of changes

• 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