Import more improvements from the Lake of Constance:
* Models/Instruments/egttemp_[LR].xml: combine into a single file, egttemp.xml,
that takes parameters for the EGT of two engines.
Alter the scale from -50 .. 600°C to 200..900°C.
* Models/Instruments/egt.rgb: alter the scale.
* Models/Lockheed1049h_Instruments.xml: pass the EGTs as parameters.