Fix multi-line math blocks losing indentation in LaTeX

Multi-line math content (e.g. \begin{align}...\end{align}) from
doxygen XML was emitted as:

    .. math:: \begin{align}
    \mathbf{v}
      = \mathbf{u} + ...
     \end{align}

RST requires directive body content to be indented. The
continuation lines at column 0 were parsed as regular text by
docutils, producing garbled LaTeX output with "Runaway argument"
and "Missing $" errors — 164 LaTeX errors on the Notation page
alone.

Fix: in visit_formula, detect multi-line math text and emit it
as a properly indented block:

    .. math::

       \begin{align}
       \mathbf{v}
         = \mathbf{u} + ...
       \end{align}

Single-line math is unchanged (stays on the .. math:: line).

After fix: `make latexpdf` LaTeX errors drop from 164 to 10.
The remaining 10 are unrelated (9 Unicode Greek characters in
source comments that pdflatex cannot render, 1 duplicate label).

Author: adcroft

docs/_ext/autodoc_doxygen/xmlutils.py

@@ -683,9 +683,14 @@ def visit_formula(self, node):
             if len(self.math_labels) > 0:
                 self.emit_math_labels()
 
-            #self.lines.append('')
             self.blank_line()
-            self.lines.append('.. math:: ' + text)
+            if '\n' in text:
+                self.lines.append('.. math::')
+                self.lines.append('')
+                for mathline in text.split('\n'):
+                    self.lines.append('   ' + mathline)
+            else:
+                self.lines.append('.. math:: ' + text)
             self.blank_line()
             # Math blocks require an explicit blank line as well?
             #self.lines.append('')