sac read/write fixes: swapping bytes, missing header field.

This PR fixes some sac-related code:

  - byte swapping should work correctly now;
  - a header field was missing in the `IDICT`dictionary, implying that this field was aways written to disk as `0`;
  - a test in `tes_core.py` was pointless.

**Please don't merge yet**, this PR is still incomplete.

 - [x] Tests are still failing because of a problem I will discuss in a comment below. [fixed]
 - [x] I would like to add a `byteorder` option to `writeSAC()`
 - [ ] Update copyright in headers and update `CHANGELOG`
Better to just set `byteorder="<"` if it defaults to little endian in any case. Clearer behavior in my opinion.