fix default slide values

[dsweber2]

Checklist

Please:

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  dajmcdon.
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Change explanations for reviewer

The actual defaults for epi_slide for before and after are NULL, so defaulting to 0 here breaks in the case of e.g. weekly data. For an example, see the test.

[dajmcdon]

@dsweber2 Can this be closed in light of #386 ?

[dsweber2]

looks like it

[dsweber2]

might be worth keeping to have a test of weekly, though probably easier to start from scratch

[dajmcdon]

Agreed. Let's rebase this on dev after #386 (changes the args to step_epi_slide anyway), then add a test for weekly.

[dshemetov]

This should raise an error if .window_size = NULL because epi_slide already does