Correct local matrix for elliptic problem.
[ML] GaussIntegration: Correct difference btw. order and number of points.

For integration of a linear function we need to evaluate the function at two points, thus linear -> first order, and n_points is 2.

Sometimes integration order was misused as number of sampling points. This is consistent now.

The fourth order method using 5 sampling points was added.
Correct debug output.
[doc] Update Doxygen.in for 1.8.6 version.

Only comments are changed and new default values added.
[ML] Correct shadowing of variables.
[style] Remove semicolons; Empty return docs; Indentation.
md to previous Integration commit
WIP
Merge pull request #6 from endJunction/2dGroundWaterFlow

Thanks for your work, Dima!