Add option to include layout json

[chrishalcrow]

Allows you to run e.g.
sigui --layout_file=my_layout.json ~/Work/my_projects/si_talk/M25_D18/kilosort4_sa
where if your json looks like this:

{
    "zone1": ["spikeamplitude"], 
    "zone2": [], 
    "zone3": ["waveform"], 
    "zone4": ["similarity"], 
    "zone5": ["unitlist"], 
    "zone6": ["correlogram"], 
    "zone7": ["spikedepth"], 
    "zone8": []
}

you get a GUI that looks like this:

Works for web too!

Also added checks to verify that the keys and values in the json file are valid (and does this check if you programatically pass a dict).

Will add docs if you agree with the PR, and after we debate #163.

[samuelgarcia]

This should be backend independant here I think.

[chrishalcrow]

oup, good catch, fixed!