various fixes for altitude in tpo readers:

various fixes for altitude in tpo readers:
for TOPO versions older than 3, and TOPO versions 3 and newer, don't use an altitude of zero for unknown as this represents a valid altitude.
for TOPO versions 3 and newer don't truncate the fractional part of altitude.
for TOPO versions 3 and newer correctly convert negative altitudes.
for TOPO versions 3 and newer change the value used to check for unknown altitudes.