Handle different SRTM versions

[jda]

Currently, we assume 1 arc-second data:

srtm/file.go

Line 19 in d8919c8

const squareSize = 3601 // 30m SRTM data is 3601 squares tall/wide

Should detect this from files instead of using a constant, and provide a way for people to determine what kind of file they are loading.