Missing parts from #83

[ThorvaldLarsen]

Requests not implemented in #83 before merge. Would eb good to have a discussion whether there is a reason not to implement if that is the choice.

I have a couple of request:

• Can we make label and unit of the .data parameter depend on demod_type? So that the label will be e.g. Imaginary (V) or Phase (degree) automatically after changing demod_type?
• Would it be possible to include an update to the ATS9360_ext driver, which has the seq_mode parameter, in this PR to reflect the new setup with settings defined in the yaml file instead? Maybe @nataliejpg has an opinion on updating it. (see New Alazar_ext driver #100)
• Getting snapshot update warnings from 0D parameters. Solved by adding snapshot_get=False in the super.init call of Alazar0DParameter. (snapshot_get = False is default for ArrayParameters so there is no problem with AlazarNDParameter.)
• With the current driver I can simultaneously take data from all channels or slices of channels. Is it also possible to take data from an arbitrary set of channels? This seems more and more useful with the current workflow at T3.

@jenshnielsen @Dominik-Vogel @nataliejpg