Client token writing is inconsistent

[TOFarmer]

Summary

If the SALClient authenticates by simply calling authenticate, the received token is assigned to SALClient.auth_token, but is not written to a file.
By contrast, if a SALClient which has not previously authenticated calls list of get then as well as authenticating, the token is written to a file.

Possible fixes

I think it would make sense to move the token writing (_write_auth_token) inside SALClient.authenticate and write the token by default using a write_token=True parameter. Then write_auth_token can be removed from _make_request, which already calls authenticate.

[TOFarmer]

It would also be beneficial to add token writing tests after this issue has been addressed.