more robust handling of voltage angle difference limits

[michaelbynum]

Summary/Motivation:

The computation of bounds for the auxiliary variables c = v_f * v_t * cos(dt) and s = vf * vt * sin(dt) had bugs. This PR uses interval arithmetic to improve the robustness of these computations and adds tests.

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[DLWoodruff]

This makes sense and it is now easier to read.