option to specify default dataset for a connection

[jefferis]

I think it would make sense to allow specification of a default dataset for a connection since the current arrangements are not so convenient when you need to work with two connections with different default datasets.

[jefferis]

There is a question of how / if to capture a default at login. In general it's probably better to use the default that you figure out from the server rather than an environment variable.

[jefferis]

Should we also error out if a bad dataset is specified? I have now opted to do so.

[jefferis]

There is a question of how / if to capture a default at login. In general it's probably better to use the default that you figure out from the server rather than an environment variable.
Opted to use the environment variable if it was congruent with the server, otherwise to ignore it.

[jefferis]

closed by #116