Handle optional m and elev coordinates when transforming between geographic and cartesian geocentric coordinates

[navispatial]

When optional m is available in source, it should be just copied to target.

Add omitElev parameter to conversions methods that result on geographic positions, this allow omitting elevation from the target:

  /// If [omitElev] is true, the optional elevation value is not calculated and
  /// it's set to null on the result geographic position. This can be useful if
  /// the elevation is near the ellipsoid surface and a non-zero elevation
  /// potentially resulted from the calculation is not wanted.

When converting from geographic to another geographic position between datums, and source do not have elevation then target should not either (even if calculation could generate one).