SMarker has new member:

`int distancematrixindex;`

Class Form keeps the Boolean `matrices_up_to_date` to check whether the
matrices have to be recalculated.

Every time a marker is *added*, the matrices' information become
irrelevant. If a marker is *removed*, the matrices don't lose their
relevance.
Rename function in class Form

changed the name of `adapt_order_smarkers` to
`process_result_distancematrix` as the name more accurately expresses
the functions', errr, _function_.