Hardcode n_frames_per_iter to 1

[mcwitt]

Motivation

• Methods to extract trajectories by replica currently assume n_frames_per_iter==1 and will break otherwise
• Since HREX now has minimal overhead, there's no clear use case for performing HREX moves less frequently than once per frame
• Hardcoding n_frames_per_iter = 1 allows removing some subtle logic that exists to handle edge cases