Refactor observation filtering functions

[athowes]

In line with the filter_obs_by_ptime() function, I think it would be more descriptive and clear to have the filter_obs_by_obs_time() function be called filter_obs_by_stime(): filter observations by the time of the secondary observation.

Perhaps there is a question as to whether we want to be implementing "simple" operations like this, but if we are then I think the rename is a good idea.

[athowes]

As a part of this issue, I propose to:

• standardise the filter_obs_by_obs_time and filter_obs_by_ptime functions (perhaps could be merged into one function with an argument for p or s),
• create unit tests for these two functions, and
• fill out the roxygen2 documentation for these two functions.

[athowes]

We have deleted the observation filtering functions in PR #434. I don't think this issue will help much if/when we readd so I suggest delete.