Data post-processing functions (general utilities for analyses)

[ha0ye]

• Making equi-timed data
  The data format uses a time column in the covariates to help organize the timing of samples. To facilitate analyses that want equi-timed data, i.e. samples that occur regularly in time, we should provide a function that will produce abundance data with explicit NAs for missing samples.

- [ ] Functions to add covariates of sine and cosine periodic signals per timespan (year) for LDATS
(should go in MATSS-LDATS or LDATS)

• interpolation
  Maybe also functions to help interpolate data?

• handle and detect non-integer timestep data Add functions to handle and detect non-integer timestep data #136
  Some methods are going to want to know about specific timestamps, and may require discrete timesteps, so some general handling would be good.