* big rework of Inverse.h:

- remove all invertibility checking, will be redundant with LU
  - general case: adapt to matrix storage order for better perf
  - size 4 case: handle corner cases without falling back to gen case.
  - rationalize with selectors instead of compile time if
  - add C-style computeInverse()
* update inverse test.
* in snippets, default cout precision to 3 decimal places
* add some cmake module from kdelibs to support btl with cmake 2.4

doc/snippets/MatrixBase_computeInverse.cpp

@@ -0,0 +1,5 @@
+Matrix3d m = Matrix3d::random();
+cout << "Here is the matrix m:" << endl << m << endl;
+Matrix3d inv;
+m.computeInverse(&inv);
+cout << "Its inverse is:" << endl << inv << endl;