Add support for special values (f.e. longitude, latitude)

[alios]

There are some special (not loco dependent) variables like longitude and latitude available.
They start with value id 400 ff.
Described in the helpful API tutorial by Matt Peddlesden attached here:
Train Simulator External Interface API.pdf

(had to attach the Document by attached here as original source seems to have disappeared http://www.engine-driver.com/article/show/10403/train-simulator-cab-desk-mock-up)