[sas-] make XptSheet rows be list instead of tuple

[midichef]

Currently XptSheet in loaders/sas.py produces rows that are tuples.

That's undesirable because as @saulpw pointed out recently, rows can't be edited if they are tuples. And also because selecting one row may cause other rows to be selected if they are interned tuples, that have the same values and ID. This PR converts the tuples to lists.

to reproduce: pip install xport==2.0.2 download brakes.xpt vd brakes.xpt then inspect any row with ^Y for pyobj-row, the status bar will say tuple.