Automatic merge of T1.5.1-1852-g24537f48e and 10 pull requests

- Pull request #570 at 362e4e7: glTF 2.0 support with PBR lighting
- Pull request #1062 at 9a76b26: Train Forces popup Window.
- Pull request #1064 at 1be1fc4: Add Train Info tab to Help window (F1)
- Pull request #1085 at 37e2817: updates key commands for Train Operations window and also Daylight Offset
- Pull request #1086 at e10390b: Add Settings Exporter tool (copy settings to INI, etc)
- Pull request #1090 at 9e0bfe8: Add null check to avoid crash on corrupt signal files
- Pull request #1092 at 344634b: Adds $gradient, $req, depart-early and passing-time to Manual
- Pull request #1082 at 5845a1a: Allow variable water level in glass gauge
- Pull request #1081 at 689494b: Brake cuts power unification
- Pull request #1091 at b0c622b: Automatic speed control

Author: openrails-bot

Source/Documentation/Manual/timetable.rst

@@ -523,8 +523,9 @@ Gradient
         rising gradient which exceeds n percent.
         However, forced acceleration is turned on if the speed of the train drops below speed s. 
         This is to ensure the train will not stall, and also that the train will restart if it has come to a stop on a 
-        gradient, e.g. due to a signal. 
-        The dimension of the speed value depends on the definition of the speed field (see manual for details of speed fields).
+        gradient, e.g. due to a signal.
+
+        The units of speed (m/s, mph, kph) depend on the definition of :ref:`the speed field<timetable-train-speed>`.
 
 
 Station Commands