Time sampling for positive pair

[alishbaimran]

Goal: be able to sample the positive pair given a "timestep". For example, sample at timestep 1 would mean sampling the nearest timepoint.

Key features:

• [ ] Specify range of timestep to sample. Timestep 1 means nearest timepoint, timestep 2 means second nearest timepoint, etc.
• [ ] Use sampling rate (should be specific depending on dataset) to determine the experimental timepoint to draw anchor pair from.
• [ ] Sample either forward or backward. We can pick a direction for now and always sample that as this theoretically shouldn't impact the prediction. If we are at the edge and don't have timepoint before or after, then sample the other way. If timepoints available both forward and backward, then always sample x (forward or backward).