fix: reset FixedDelay state at the beginning of each run

[ToddFincannonEI]

Fixes #285

The solution detects a change in the delay time and reallocates the data buffer accordingly. This required a bit of refactoring to prevent duplicate calculation of the number of buffer elements and the buffer size.

In the issue I forgot to mention that this situation only comes up when an SDEverywhere model is run more than once. Inputs to subsequent runs can cause a recalculation of the delay time argument to FIXED DELAY. Because the buffer is dynamically allocated, it is preserved across model runs. It needs to be reset on subsequent runs even if the buffer size has not changed. The solution handles that too.

[chrispcampbell]

This looks good to me. I wish there was an automated test for this but understand that we don't have the infrastructure at the moment to have tests that verify results for multiple run iterations. (I'm planning to add JS-level integration tests that should make that sort of thing possible, but not quite there yet.) Anyway! I'll merge this now.