Fix calculation of STIX pointing in HPC to imaging demo

[DanRyanIrish]

This PR corrects how the HPC coordinate of the centre of the STIX pointing is calculated in the imaging demo. While the pre-existing calculation is approximately valid in the demo's case, it is not valid for substanital roll angle. The approach given in the PR should be valid for all roll angles.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 69.52%. Comparing base (a694ede) to head (fc8a436). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #112 +/- ## ========================================== + Coverage 68.92% 69.52% +0.60% ========================================== Files 31 31 Lines 1847 1887 +40 ========================================== + Hits 1273 1312 +39 - Misses 574 575 +1 ```

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