Fixed a couple of crash scenarios.

* MapViewController.cs: Changed overload contructor that took lat,
  lon, and locationName and made it a public method to set the data on
  an existing object. IOW, now only using one MapViewController where
  before I was making a new one with each map request. That was
  causing a crash. 

* StationsTableView.cs: Added class reference to the MapViewController
  to use all the time. Also, enabling and disabling the Map
  Selection/Location button to avoid a crash that occurred if you hit
  the button while the map view was being dismissed. 

* TidesTableView.cs: Added ViewDidAppear method to reenable the Map
  Selection/Location button when the TidesTableView becomes visible.