Refactoring: Made dropping features part of API

Instead of just printing, we allow people to access the information
programmatically too.

core/src/main/kotlin/se/lth/cs/util/Features.kt

@@ -207,12 +207,21 @@ class FeatureSet(vararg features : Feature) : ArrayList<Feature>() {
         val featuresChecked = this.partition { it.isSupported() }
 
         if (!featuresChecked.second.isEmpty()) {
-            System.err.println("[WARNING]: Some features have been dropped, they are not supported on this machine: ${featuresChecked.second}")
+            for (f in featuresChecked.second) {
+                dropFeature(f)
+            }
         }
 
         return FeatureSet(*featuresChecked.first.toTypedArray())
     }
 
+    val droppedFeatures : MutableList<Feature> = mutableListOf()
+
+    fun dropFeature(feature : Feature) {
+        System.err.println("[WARNING]: Feature '$feature' is not supported on this machine, it will not be gathered.")
+        droppedFeatures.add(feature)
+    }
+
     fun accept(visitor : FeatureVisitor) {
         visitor.visitFeatureSet(this)
     }