Incorrect zero check when parsing latitude/longitude/altitude

Via [this post](https://community.opendronemap.org/t/several-issues-found-in-srt-parsing-and-gps-altitude-handling/25790),

> File: opendm/video/srtparser.py
> The following condition compares a string to an integer and therefore always evaluates to True:
> `lambda v: float(v) if v != 0 else None`
> Suggested fix:
```
fv = float(v)
return fv if fv != 0 else None
```