Use "align" environment to wrap math equations instead of "gather"

Using align brings the benefit when multiple math equations are in one
math directive, alignment is provided. We also use "aligned" to wrap the
equations since in this way all the equations will be given only one
label/tag, while previously only the first equation is labeled.

tests/roots/test-ext-math/conf.py

@@ -1,3 +1,6 @@
 # -*- coding: utf-8 -*-
 
 master_doc = 'index'
+
+latex_documents = [
+    (master_doc, 'test.tex', 'Math Extension Testing', 'Sphinx', 'report')]

tests/roots/test-ext-math/index.rst

@@ -8,3 +8,11 @@ Inline :math:`E=mc^2`
 Second math
 
 .. math:: e^{i\pi}+1=0
+
+Multi math equations
+
+.. math::
+
+   S &= \pi r^2
+
+   V &= \frac{4}{3} \pi r^3