Eliminated all the calls to std::find()

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.