Changes

pySim/transport: add abstract get_atr method to LinkBase

The implementations that inheret from the LinkBase class are expected to
implement a get_atr method. This method is mandatory, since it is one of
the most basic functionalities of pySim to display an ATR. Also the ATR
is sometimes needed to distinguish between different card models.

The modem_atcmd and calypso implementation completely lack the get_atr
method. Apparantly it is not possible to get an ATR in those
environments, so lets add a dummy method there.

Related: OS#6322
Change-Id: I4fc020ca45658af78e495a5c1b985213f83cbb50