Make hardcoded error values configurable.

Make hardcoded error values configurable.
This patch introduces the optional elements
<config>
   <spin-thresh>0.8</spin-thresh>
   <max-roll-error-deg>40</max-roll-error-deg>
   <max-pitch-error-deg>12</max-pitch-error-deg>
</config>
under the corresponding /instrumentation/attitude-indicator element.
The values are initialized to the shown default values if omitted.
These are the values of the former hardcoded constants.