added maxItems so that length of "coordinates" array is [2,3] (#336)

Co-authored-by: Marc Lichtman <[email protected]>

Author: gmabey

sigmf-schema.json

@@ -111,6 +111,7 @@
             "coordinates": {
               "type": "array",
               "minItems": 2,
+              "maxItems": 3,
               "items": {
                 "type": "number"
               }
@@ -214,6 +215,7 @@
                   "coordinates": {
                     "type": "array",
                     "minItems": 2,
+                    "maxItems": 3,
                     "items": {
                       "type": "number"
                     }