Add Maximum for calculation sensor uses

Add a type - Maximum for calculation sensor uses.

There are too many NVMe temperature sensors of the same type.
The fan speed regulation strategy only requires a maximum hard
disk temperature, which can be used as a PID parameter.

The sensor list does not need to know all the NVMe temperatures,
but only the maximum temperature of the NVMe.

For example:
In my machine has two sets of NVMes, one is 24 NVMe,
the other is 4 NVMe disks, but only the maximum temperature
of the two sets is required to participate in PID
and display to the customer.

Tested:
Need to add code to photoshop-virtual-sensor to calculate sensor value.

usage like:
in entity-manager/baseboard.json:
add virtual sensor use Type=Maximum:
```
    {
        "Exposes": [
            {
                "Name": "NVMe MAX Temp",
                "Sensors": [
                    "NVMe 0 Temp",
                    "NVMe 1 Temp",
                    "NVMe 2 Temp",
                    "NVMe 3 Temp",
                    "NVMe 4 Temp"
                ],
                "Thresholds": [
                    {
                        "Direction": "greater than",
                        "Name": "upper critical",
                        "Severity": 1,
                        "Value": 116
                    },
                    {
                        "Direction": "less than",
                        "Name": "lower critical",
                        "Severity": 1,
                        "Value": 1
                    }
                ],
                "Type": "Maximum",
                "Units": "DegreesC"
            }
        ],
        "Logging": "Off",
        "Name": "NVMe MAX",
        "Probe": "TRUE",
        "Type": "NVMe"
    }
```

Signed-off-by: Tao Lin <lintao.lc@inspur.com>
Change-Id: Id0f2c465200301c95bf54b19f11a5962d662d164
diff --git a/schemas/virtual_sensor.json b/schemas/virtual_sensor.json
index 3584489..77c39c9 100644
--- a/schemas/virtual_sensor.json
+++ b/schemas/virtual_sensor.json
@@ -52,7 +52,7 @@
                         "The type of calculation this sensor uses."
                     ],
                     "type": "string",
-                    "enum": ["ModifiedMedian"]
+                    "enum": ["ModifiedMedian", "Maximum"]
                 },
                 "Units": {
                     "description": [