[sumitsaraswat362]

Fixes #152

Description

This PR addresses the bug reported in #152 where FieldLine.b_along_fline was returning different values for the same magnetogram depending on its rolling offset.

Cause:
In pfss.Output.get_bvec, the input SkyCoord was being transformed to the model's coordinate frame, but the result of coords.transform_to() was not being assigned back (it returned a copy). Additionally, the longitude passed to the internal interpolation function _brgi was not properly adjusted for the _lon0 offset (like it is handled inside tracing.py).

Fix:

• Updated the transformation to save the transformed SkyCoord in-place.
• Applied the coords.lon - (self._lon0 - 180 * u.deg) shift before generating the cartesian vector and interpolating.
• Reassigned the shifted coordinates back to a new SkyCoord so the rotation matrices (which convert the cartesian output back to spherical coordinates) also operate on the correctly shifted longitude.

This aligns the behavior of get_bvec with the coordinate handling inside the PerformanceTracer, ensuring consistent B vectors regardless of the magnetogram roll.

[sumitsaraswat362]

Closing this PR as I am focusing on other contributions.