Feature 2708 mvmode merge flag default (#2721)

* mods to use the multivar mode default config (when in multivar mode), and to check for problematic merge config settings

* added more meaningful error and debug messages

* Modified documentation to better describe multivariate mode including new default values

* Bug fix for unit test

* cleanup and fix a bug in which the default output path was not set in multivar mode

* Changed multivar_name value to 'Super' in MODEMultivarConfig_default

data/config/MODEMultivarConfig_default

@@ -89,7 +89,7 @@ fcst = {
    filter_attr_thresh = [];
    merge_thresh       = >=3.5;
    merge_flag         = NONE;
-   multivar_name = "ALPHA_BETA_GAMMA";
+   multivar_name = "Super";
    multivar_level = "LO";
 }
 obs = fcst;