fix!: change Series.getValueAtTime to work with non-1 save frequency

[chrispcampbell]

Fixes #259

BREAKING-CHANGE: This renames Outputs timeStart/timeEnd to startTime/endTime and adds a saveFreq argument (defaults to 1).

This primarily fixes an issue with Series.getValueAtTime where it previously assumed that the output data had a save frequency (SAVEPER) of 1. It now handles arbitrary save frequency values, and does a linear search instead of assuming that it can jump to a particular index. (Binary search can come later.)

To make this work, I added a saveFreq argument to the Outputs constructor. While I was at it, I renamed the timeStart and timeEnd fields on Outputs to be startTime and endTime to be consistent with other uses. (These fields aren't typically accessed but will remain public, and thus the breaking change.)

Note that the rest of the runtime and build packages still need more work to handle non-1 SAVEPER values. That work will be completed soon under separate issue #291.

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fix!: change Series.getValueAtTime to work with non-1 save frequency

BREAKING-CHANGE: The timeStart and timeEnd properties in the Outputs class in the @sdeverywhere/runtime package have been renamed to startTime and endTime, and there is an additional saveFreq argument for the Outputs constructor (which defaults to 1).
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