xtf can't be read correctly

[hanhaixingxing]

The XTF file that write by your own can't be read by plot_sonar_bathy

[oysstu]

Fixed e86b8a7

[Thomas-Mccabe]

The plotter won't read anything that you make that has n_samp being anything other than 20.

It produces an error:
RuntimeError: XTF packet does not start with the correct identifier (0xFACE).

[oysstu]

This happened because an index was automatically created when read was called, this was stored next to the xtf-file and used automatically if present. I originally did it this way to parse huge xtf files, in order to speed up selective reading of a subset of datagrams. The index didn't contain any hash or other checks, and would assume that a file of the same base name was the same file as had been indexed.

The write example creates an example output with the same name repeatedly, and pyxtf would try to read it using the index for a previous file version.

I have disabled indexing by default in 861a5f5. This should resolve the issue for normal use.