Make database.capacity writable

docs/api/reference-image-database.md

 
 ### capacity
 
-`database.capacity: number, read-only`
+`database.capacity: number`
 
 The maximum number of reference images that can be stored in this database.
 
+*Note:* this property is writable since version 0.2.1 (experimental).
+
 ## Methods
 
 ### add

src/trackers/image-tracker/reference-image-database.ts

 import { IllegalArgumentError, IllegalOperationError } from '../../utils/errors';
 
 /** Default capacity of a Reference Image Database */
-const DEFAULT_CAPACITY = 100;
+const DEFAULT_CAPACITY = 100; // this number should exceed normal usage
+                              // XXX this number may be changed (is 100 too conservative?)
+                              // further testing is needed to verify the appropriateness of this number;
+                              // it depends on the images, on the keypoint descriptors, and even on the target devices
 
 /** Generate a unique name for a reference image */
 const generateUniqueName = () => 'target-' + Math.random().toString(16).substr(2);
 
     /**
      * Maximum number of elements
+     * Increasing the capacity is considered experimental
      */
-    /*
     set capacity(value: number)
     {
         const capacity = Math.max(0, value | 0);
 
         if(this.count > capacity)
-            throw new IllegalArgumentError(`Can't set the capacity of the database to ${this._capacity}: it currently stores ${this.count} entries`);
+            throw new IllegalArgumentError(`Can't set the capacity of the database to ${capacity}: it currently stores ${this.count} entries`);
 
         this._capacity = capacity;
     }
-    */
 
     /**
      * Iterates over the collection