Support for resources list (#100)

* Separate functionalities for retrieving collection resources

* Chapter and chunk titles

* Revert "Chapter and chunk titles"

This reverts commit 799b208.

* Revert "Separate functionalities for retrieving collection resources"

This reverts commit bc7bc96.

* Create RxUtils.kt

* Add kdoc to RxUtils.mapNotNull

src/main/kotlin/org/wycliffeassociates/otter/common/utils/RxUtils.kt

@@ -0,0 +1,14 @@
+package org.wycliffeassociates.otter.common.utils
+
+import io.reactivex.Observable
+
+/**
+ * Apply the given function as with `map()`, but remove any resulting nulls from the
+ * observable stream.
+ *
+ * This is how `observable.map(f).filter { it != null }` should behave, but it keeps
+ * nulls from ever appearing in an Observable, which would cause a crash in that
+ * two-step map/filter example.
+ */
+fun <T, R:Any> Observable<T>.mapNotNull(f: (T) -> R?): Observable<R> =
+    concatMapIterable { listOfNotNull(f(it)) }