formalize states of `Knit` operation

[t0mpr1c3]

At the moment, the Knit operation is governed by multiple state variables that control program flow in procedures such as knit(). These variables include the following members of class OpKnit:

  bool m_lineRequested;
  bool m_lastLineFlag;
  bool m_firstRun;
  bool m_workedOnLine;

plus (m_sOldPosition == position) and so on.

The imperative code works fine but if we want to pursue a more OO programming style it is possible to abstract some or all of these into a sequence of operations governed by the finite state machine.

My suggestion is to leave things as they are for now, but think about making changes if we want to do #105 which will add further complexity to the program flow.

[t0mpr1c3]

We can actually get rid of one of these state variables right away by moving the first run from OpKnit::knit() to OpKnit::begin().