Allow different semantics for on-context triggers

[Steffen Zschaler]

On-context triggers should fire either because a change has occurred in the cell's context or through a periodic evaluation of the entire field (constructing a new generation). This way, we can support different kinds of grid games such as mine sweeper or game of life.

For now, implement this through an additional option at the level of a game specification:

game GameOfLife {
  ...

  contextsTriggerGenerationally
}

[Steffen Zschaler]

All done.