Fix initial state of the Modify interaction

Features that are in the features collection need to be added to the R-Tree.
Create the R-Tree in the Modify constructor

In this way features can be added/removed to/from the R-Tree even when the interaction does not have a map.
Make the Modify interaction more robust

This prevents a JavaScript error when features are added while the interaction does not have a map.
Merge pull request #1922 from elemoine/modify

Fixes to modify interaction