Reference Sampling Approach

Currently, references are sampled as follows

```
xrefs = x_samples[x_indices[:Nx]]
x_samples = x_samples[x_indices[Nx:]]
```

```
yrefs = y_samples[y_indices[:Ny]]
y_samples = y_samples[y_indices[Ny:]]
```

This can be problematic if the samples are not randomized, thus the regions are biased to certain modes.

Suggestion:
- Modify the region sampling to be random instead of by slice