parsePaths performance

This MR will replace the use of TList (both internally and as the returned data structure) with std::vector and a new PathTerm structure.

Before, the return value of parsePaths looks like:

TList(TList(TList()))

• The elements of the outer-most TList are the sets of alternative paths (by default just 1)

• The elements of the middle TList are the different terms in the path arithmetic

• The elements of the inner TList are always the same in number (3) and represent the path, factor, and statCor for the path term

After, the return value of parsePaths will look like:

Vector(Vector(PathTerm))

helps address #71 (closed)