docs: indent struct-field descriptions to 4 spaces

[edkerk]

Problem

Building the RAVEN documentation site emits 18 warnings of the form:

griffe: analysis/sampleCHRR.m:42: Confusing indentation for continuation line 41 in docstring, should be 4 spaces, not 2
griffe: gapfilling/gapFillTopological.m:45: Confusing indentation for continuation line 44 in docstring, should be 4 spaces, not 2

Both come from the same pattern — a struct-valued return whose fields are
listed at 2 spaces:

% info : struct with fields:
%   .nDimensions  — dimension of the sampled flux polytope

In numpydoc style, a description continuation must be indented 4 spaces
relative to the entry name. At 2 spaces the field list is not reliably read as
part of the entry, so it can render outside it on the generated API page.

Fix

Indent the field lines to 4 spaces, and give both entries the plain type
struct. That matches analysis/compareFluxes.m, which documents a struct
return the same way and produces no warnings:

% result : struct
%     .turnedOn  — cell, rxn IDs newly active in fluxes2

Documentation only — no functional change.

Verification

With these two files in place, mkdocs build --strict in the docs site reports
zero "Confusing indentation" warnings, down from 18.

Found while writing the flux-variability and gap-filling pages of the new
user guide.

[github-actions]

Function test results

300 tests   274 ✅  1m 18s ⏱️
 25 suites   26 💤
  1 files      0 ❌

Results for commit 19354cc.