Add min elevation/elevation mask config property

[jclark]

Many receivers support an elevation mask configuration option which specifies the min elevation angle for a satellite to be used in the navigation solution.

This seems a common option. It's available even in u-blox 6 series (as minElev in cfg-nav5).

For Unicore it is part of the MASK command, which we have to implement anyway for EnabledSignals.

But should we call it

• MinElevation, or
• ElevationMask

?

For unicore it can be fractional, so we should represent it as a gpsprot.Angle.

[jclark]

Decided in MinElevation