Consider a rename of epi_slide / epix_slide

[dshemetov]

• consider a rename of epi_slide / epix_slide (but let's try not to get stuck here)

Originally posted by @dshemetov in #477 (comment)

Two reasons to consider a rename:

• the details of the functionality has drifted for both functions, with them becoming more focused and specific; this may allow us to be more descriptive in a new name
• their naming similarity was maintained because we saw them as the same slide operation but on two different time dimensions (time_value and version/issue) and this led to wanting to keep their interfaces (and defaults) similar; as we came to use these functions more, we discovered that the defaults for these should be different (e.g the sliding window needs to support different alignments and not have a default size in epi_slide, while only a right-aligned and Infinite window size makes sense in epix_slide) and that their use cases are different enough to eclipse the underlying conceptual similarity