Typos.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1536072 13f79535-47bb-0310-9956-ffa450edef68
Added SparseGradient to deal efficiently with numerous variables.

This new class is devoted to differentiation computation when the number
of variables is very large but most computations depend only on a few of
subset of the variables.

Thanks to Ajo Fod for the contribution.

JIRA: MATH-1036

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1536073 13f79535-47bb-0310-9956-ffa450edef68