"CVector" and "RVector" represent the column and the row of matrix.
"(*RVector).IsUndefined" returns "true" if some undefined operation applies to the vector.
"NewRVector" creates a row vector from the specified row of a matrix.
Fix: Call "(Matrix).Shape" and replace the reciever with "*CVector".
"(*RVector).Get" obtains its element through its base matrix.
"(*RVector).IsUndefined" also returns "true" if the base matrix is undefined.
"(*RVector).Update" requires three arguments. The last one is a new "value" of vector.
Implement methods similar to "*RVector"'s methods for "*CVector".
Merge branch 'feature/vector' into master