use methods to specify which report rather than string constants

* MultiReporter
    deleted. direct method calls are a better way to specify the kind of report desired
    getReports was never used to choose from the array of strings. no use case for MultiReporter.
* ArchiveUtils
    writeReportToString was not used, and it used the now deleted MultiReporter#reportTo
* DevUtils, Frontier, ToePool, AbstractFrontier, WorkQueueFrontier
    MultiReporter -> Reporter and make methods that implemented named multireport public
* FrontierNonemptyReport, FrontierSummaryReport
    call the appropriate report method instead of passing a string constant

commons/src/main/java/org/archive/util/DevUtils.java

@@ -55,8 +55,8 @@ public class DevUtils {
         StringWriter sw = new StringWriter();
         PrintWriter pw = new PrintWriter(sw); 
         final Thread current = Thread.currentThread();
-        if (current instanceof MultiReporter) {
-            MultiReporter tt = (MultiReporter)current;
+        if (current instanceof Reporter) {
+            Reporter tt = (Reporter)current;
             try {
                 tt.reportTo(pw);
             } catch (IOException e) {