big huge changes, so i dont remember everything.

* renaming, e.g. LU ---> FullPivLU
* split tests framework: more robust, e.g. dont generate empty tests if a number is skipped
* make all remaining tests use that splitting, as needed.
* Fix 4x4 inversion (see stable branch)
* Transform::inverse() and geo_transform test : adapt to new inverse() API, it was also trying to instantiate inverse() for 3x4 matrices.
* CMakeLists: more robust regexp to parse the version number
* misc fixes in unit tests

doc/snippets/class_FullPivLU.cpp

@@ -0,0 +1,20 @@
+typedef Matrix<double, 5, 3> Matrix5x3;
+typedef Matrix<double, 5, 5> Matrix5x5;
+Matrix5x3 m = Matrix5x3::Random();
+cout << "Here is the matrix m:" << endl << m << endl;
+Eigen::FullPivLU<Matrix5x3> lu(m);
+cout << "Here is, up to permutations, its LU decomposition matrix:"
+     << endl << lu.matrixLU() << endl;
+cout << "Here is the L part:" << endl;
+Matrix5x5 l = Matrix5x5::Identity();
+l.block<5,3>(0,0).part<StrictlyLowerTriangular>() = lu.matrixLU();
+cout << l << endl;
+cout << "Here is the U part:" << endl;
+Matrix5x3 u = lu.matrixLU().part<UpperTriangular>();
+cout << u << endl;
+cout << "Let us now reconstruct the original matrix m:" << endl;
+Matrix5x3 x = l * u;
+Matrix5x3 y;
+for(int i = 0; i < 5; i++) for(int j = 0; j < 3; j++)
+  y(i, lu.permutationQ()[j]) = x(lu.permutationP()[i], j);
+cout << y << endl; // should be equal to the original matrix m