* 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/compile_snippet.cpp.in

@@ -1,10 +1,13 @@
 #include <Eigen/Core>
 #include <Eigen/Array>
 #include <Eigen/LU>
+
 USING_PART_OF_NAMESPACE_EIGEN
 using namespace std;
+
 int main(int, char**)
 {
-${snippet_source_code}
-return 0;
+  cout.precision(3);
+  ${snippet_source_code}
+  return 0;
 }