Add tests for FileVerifier, DirectoryVerifier, and DateTimeVerifier (closes #1563)

include/openspace/documentation/verifier.h

@@ -174,6 +174,8 @@ private:
  * refers to an existing file on disk.
  */
 struct FileVerifier : public StringVerifier {
+    FileVerifier();
+
     TestResult operator()(const ghoul::Dictionary& dict,
         const std::string& key) const override;
 
@@ -185,6 +187,8 @@ struct FileVerifier : public StringVerifier {
 * refers to an existing directory on disk.
 */
 struct DirectoryVerifier : public StringVerifier {
+    DirectoryVerifier();
+
     TestResult operator()(const ghoul::Dictionary& dict,
         const std::string& key) const override;
 
@@ -196,6 +200,8 @@ struct DirectoryVerifier : public StringVerifier {
  * a valid date time
  */
 struct DateTimeVerifier : public StringVerifier {
+    DateTimeVerifier();
+
     TestResult operator()(const ghoul::Dictionary& dict,
         const std::string& key) const override;