Fix bug of ignoring the step size of a parameter

The step size attribute of a parameter is always ignored because there
is no converter that converts String to Number. The bug is fixed by
parsing the String to double. Similar bug exists in the script editor
and has been fixed.

See also:
c0ffe39
#205

Author: LeonYang5114

src/main/java/org/scijava/command/CommandModuleItem.java

@@ -145,7 +145,14 @@ public T getMaximumValue() {
 
 	@Override
 	public Number getStepSize() {
-		return tValue(getParameter().stepSize(), Number.class);
+		final String value = getParameter().stepSize();
+		try {
+			final double stepSize = Double.parseDouble(value);
+			return stepSize;
+		}
+		catch (final NumberFormatException exc) {
+			return tValue(value, Number.class);
+		}
 	}
 
 	@Override