HeterodynedPulsarModel.c: fix frequency updates to new epoch

Detailed Description

In the XLALHeterodynedPulsarPhaseDifference function in HeterodynedPulsarModel.c, if the two sets of phase evolution parameters for the the difference is being calculated have different period epochs (the PEPOCH parameter), it attempts to update the frequencies to be at the same epoch. There are a couple of bugs in this that are fixed in this MR:

• the time difference for higher order terms (higher than F1) got raised to an additional power for each term;
• the stored epoch value never got updated correctly.

This MR is quasi-related to !2451 (merged) in that it's in the same part of the code and was found when investigating that issue. This function is currently only used via CWInPy, and a test if the correctness is being included in cwinpy/cwinpy!200 (merged).

API Changes

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• These changes do not modify the API.
• These changes do modify the API, and are backwards compatible.
• These changes do modify the API, and are backwards incompatible.

For examples of changes that do not modify the API and/or are considered backwards (in)compatible, please see the contributing guide.

Justification for Backwards Incompatible Changes

Review Status

cc @david-keitel @karl-wette

added lalpulsar label

assigned to @matthew-pitkin

requested review from @david-keitel, @evan-goetz, @john-whelan, and @karl-wette

approved this merge request