Merge branch 'feature-wallDistanceCalcInterval' into 'develop'

ENH: wallDist - added option to evaluate every XXX steps

Added functionality to update the wall distance every XXX steps

Note: only applies to movePoints() - topology change bypasses the update interval and triggers a re-calculation
Syntax:
```
wallDist
{
    method          ...
    updateInterval 5; // optional - default is 1
}
```

Test case: [mixerVesselAMI2D.tgz](/uploads/c0bee1decc0337018272f3566b6a4416/mixerVesselAMI2D.tgz)

See merge request !62