Add convenience method to check if free space is below its threshold

Refers to #20

ATxCommon/Serializables/DriveToCheck.cs

@@ -22,5 +22,13 @@ namespace ATxCommon.Serializables
         /// </summary>
         [XmlIgnore]
         public long FreeSpace { get; set; } = -1;
+
+        /// <summary>
+        /// Convenience method to check if this drive's free space is below its threshold.
+        /// </summary>
+        /// <returns>True if free space is below the threshold, false otherwise.</returns>
+        public bool DiskSpaceLow() {
+            return FreeSpace < SpaceThreshold * Conv.GigaBytes;
+        }
     }
 }
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