Add hierarchical statistics support

This change adds hierarchical federated statistics support.

The existing `StatisticsController` outputs global statistics in a flat
hierarchy. There are usecases where global statistics are required to be
generated as per the given hierarchy configuration where NVFlare clients
can be specified to belong to a particular hierarchy.

The new class `HierarchicalStatisticsController` is added to support
hierarchical statistics. It is derived from `StatisticsController` and
takes additional argument `hierarchy_config` for hierarchy specification
file providing details about all the clients and their hierarchy.

Example hierarchy config file contents:

    {
        "Manufacturers": [
            {
                "Name": "Manufacturer-1",
                "Orgs": [
                     {
                         "Name": "Org-1",
                         "Sites": ["Site-1"]
                     },
                     {
                         "Name": "Org-2",
                         "Sites": ["Site-2"]
                     }
                ],
            },
            {
                "Name": "Manufacturer-2",
                "Orgs": [
                    {
                        "Name": "Org-3",
                        "Sites": ["Site-3", "Site-4"]
                    }
                ]
            },
        ]
    }

The above hierarchy config specifies three level hierarchy for four NVFlare
clients named 'Site-1', 'Site-2', 'Site-3' and 'Site-4'. And the
generate global statistics output will in hierarchical format like below. At
each hierarchical level, global statistics are caulculated whereas local
statistics are reported at the last hierarchical level.

{
    "Global": {
        <Global stats>
    },
    "Manufacturers": [
        {
            "Name": "Manufacturer-1",
            "Global": {
                <Manufacturer level stats>
            },
            "Orgs": [
                {
                    "Name": "Org-1",
                    "Global": {
                        <Org level stats>
                    },
                    "Sites": [
                        {
                            "Name": "Site-1",
                            "Local": {
                                <Local stats>
                            },
                        },
                        {
                            "Name": "Site-2",
                            "Local": {
                                <Local stats>
                            },
                        }
                    ],
                },
            ]
        },
    ],
}

The number of hierarchical levels and the hierarchical level names are
automatically calculated from the given hierarchy config. Any number of
hierarchical levels are supported.

This change also adds unit test to test hierarchical global statistics
upto four hierarchical levels.

nvflare/app_common/app_constant.py

@@ -180,6 +180,8 @@ class StatisticsConstants(AppConstants):
     STATS_2nd_STATISTICS = "fed_stats_2nd_statistics"
 
     GLOBAL = "Global"
+    LOCAL = "Local"
+    NAME = "Name"
 
     ordered_statistics = {
         STATS_1st_STATISTICS: [STATS_COUNT, STATS_FAILURE_COUNT, STATS_SUM, STATS_MEAN, STATS_MIN, STATS_MAX],