Simplify for loops

[fwesselm]

I accidentally found two for loops in src/simplex/HEkkDualRow.cpp that could be rewritten as ranged-based for loops, which would simplify the code, I think.

[jajhall]

I've never used iterators, so can't comment on the correctness of the change. What's the gain in making the change?

[jajhall]

Since both changes occur in lines that are heavily used, any errors would manifest themselves immediately.

I still wonder what's gained by changing the code in this way

[fwesselm]

Since both changes occur in lines that are heavily used, any errors would manifest themselves immediately.

I still wonder what's gained by changing the code in this way

Ranged-based for loops were introduced with C++11 (and are used in the presolve code, for example).

The code here iterates over the entire std::set<HighsInt> freeList. I think this change makes the code syntactically simpler and easier to read and understand. Of course, there won't be performance gains.