ADD: Full support for the axle_load feature:
* Specify an axel_load=X value in a vehicle's .dat file where X is a vehicle's maximum axle load in tonnes
* It is that maximum axle load that will be the weight limit for ways and tunnels
* Bridges will instead use the convoy's total weight (but adjusted to take into account the maximum part of the convoy that will be on the bridge at any given time)
* Specify an "axles=X" value in a vehicle's .dat file to calculate the axle weight automatically from the vehicle's empty weight (assuming even loading)
* The default if no value is specified for axel_load is a load of 1/2 of the weight (assumes 2 axles, load evenly distrubuted, empty weight)
* The value is in *tonnes*, not kg as for vehicle empty weight.
* Axle weights do not depend on the convoy's load
* Total convoy weight for bridges does depend on the convoy's load
FIX: Minor error in a comment in simuconf.tab