Fix negative radar elevations being reported as large possitive elevations

[AdenKoperczak]

Properly report negative level II elevations.

Fix was based on this note from ICD FOR RDA/RPG - Build RDA 23.0/RPG 23.0 (PDF) section 3.2.4.3.

NOTE: Elevation angles greater than 90 degrees will be interpreted as a negative angle and the actual elevation angle will be computed as the angle value minus 360 degrees.

I moved the conversion logic from Ar2vFile::GetElevationScan to Ar2vFileImpl::IndexFile. This mainly causes the negative elevations to be sorted before the positive ones, rather than at the end. It may take more memory to store the elevations as floats rather than uint16_t's, but there are only a few elevation records and I believe only a few Ar2vFile objects should exist at any time. (It also decreases the number of conversions that happen, but I do not believe that is a huge deal.) I could convert the angle to a int16_t in Ar2vFileImpl::IndexFile, then to a float in Ar2vFile::GetElevationScan, but I thought that would not be worth the complexity.