file_writers.py can be optimized for file system I/O

[texadactyl]

On each DAT or LOG write, the sequence is:

1. open
2. write
3. close

It makes more sense to open once at object instantiation time and close once when the object is reclaimed or the file user makes an explicit close() call.

[texadactyl]

Needs more work. Some subtleties in find_doppler.py need to be accommodated.

[texadactyl]

Withdrawn. Different focus for optimization in flight.