hp-3457a: Implement basic configuration and sampling

hp-3457a: Implement basic configuration and sampling

KNOWN ISSUES:
- When sampling with 100 NPLC, the poll function will timeout a few
times before the first sample is acquired. Increasing the timeout
passed to sr_scpi_source_add() will cause all the other commands to
be processed slowly, producing a sampling rate of about one sample
every ten seconds.
- Support for plug-in cards (44491A and 44492A) is not implemented.
- Support for AC, AC+DC and four-wire resistance measurements is not
implemented.
- Support for configuring the frequency measurement source is not
implemented.