Correctly incorporate correlated noise in WidebandDownhillFitter + Plot whitened DM residuals in pintk

[abhisrkckl]

This PR implements the correct handling of DMGP in WidebandDownhillFitter based on https://arxiv.org/abs/2505.05274

The major changes are as follows.

1. WidebandDownhillFitter now uses the wideband design matrix computed by TimingModel.wideband_designmatrix instead of constructing it inside the fitter.
2. WidebandDownhillFitter now uses the wideband Ndiag computed by TimingModel.scaled_wideband_uncertainty instead of constructing it inside the fitter.
3. WidebandDownhillFitter now computes the wideband residuals using WidebandTOAResiduals.calc_wideband_resids instead of doing it inside the fitter.
4. GLS fitting now incorporates DMGP correctly. Earlier, only DMX was supported.
5. WidebandDownhillFitter and WidebandState now have a structure more like other downhill fitter and state classes.

The GLS fitting expression is

where y, N, and M are the wideband residuals (2ntoa), wideband Ndiag (2ntoa x 2ntoa), and the wideband design matrix (2ntoa x nparam). Phi is the same as in narrowband timing (it contains the prior variances).

Also implements a new option to plot whitened DM residuals in pintk.

[abhisrkckl]

Whitened DM resids plot in pintk:

[dlakaplan]

Hi, @abhisrkckl . I don't know the correlated-noise code well enough to really review this. Maybe @rossjjennings or @demorest or somebody else can look more closely?

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.60%. Comparing base (1324586) to head (0ae6cf2).

Files with missing lines	Patch %	Lines
src/pint/pintk/plk.py	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1968      +/-   ##
==========================================
+ Coverage   70.57%   70.60%   +0.03%     
==========================================
  Files         110      110              
  Lines       25775    25730      -45     
  Branches     4060     4059       -1     
==========================================
- Hits        18190    18167      -23     
+ Misses       6443     6415      -28     
- Partials     1142     1148       +6     

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