Add boundary condition options #3
Description
We sample in the whole real space currently with no well-defined boundary conditions (e.g., periodic, reflective). Options for adding some boundary conditions are essential. One of the ideas is implementing a kernel with boundaries for the local sampler without modifying anything in the global part. In this case, the boundary features (e.g., periodic) will appear in the local sampler and the normalising flow will still consider the whole real space but seeing only samples from the bounded region. Global proposal may leak out of the boundary but those can be suppressed by the prior.