Separate out image data from point cloud data

[lizliz]

The PointCloud submodule currently has both point clouds (i.e., a bunch of points $x_1,\cdots,x_N \in \mathbb{R}^d$) and images/functions (i.e. a function like e.g. a Gaussian defined on a region $f: \mathbb{R}^d \to \mathbb R$). The latter should be separated out into their own .py file.