Eliminated all the calls to std::find()

This is done by keeping indexToRow vector, that keeps the information,
opposite to those kept by N and B. That is, while N and B help to
determine which variable corresponds to given column in column-vector c
or row in matrix b, indexToRow helps to determine the corresponding
row/column for a given variable.

At this point, I'm waiting for comments from pull request reviewer and
not working on any upgrades. Comments are appreciated, as usual.

modules/optim/include/opencv2/optim.hpp

@@ -47,7 +47,6 @@
 
 namespace cv{namespace optim
 {
-    using namespace std;
 //!the return codes for solveLP() function
 enum
 {