scpi-dmm: model and MQ dependent delay when switching functions

Some models are slow to switch meter functions to specific MQs. This was
observed with 34405A and AC V/I modes, where USB connections timed out
and suffered from pipe errors.

Add an optional delay after "CONF:<mode> [<range>]" SCPI requests. The
duration of the delay is per model, the use of the delay is per MQ on
that model. The 200ms delay for 34405A was found by experimentation.

Speaking about 34405A: Only the "INIT" request in temperature mode
remains as an issue, which is not covered by this commit's approach.