Add basic support for plotting preshaped qcodes data directly

[jenshnielsen]

This will require microsoft/Qcodes#2264 and qcodes 0.20.0 release to actually work

[astafan8]

i think this is good :) it was tough to review though because even though the changes are minimal, it does take time to understand what happens where and when and where shape is actually used, etc. :)

[astafan8]

it seems to me that datadict_to_meshgrid could use some optimization/improvement so that it does not recreate arrays if existing shape is to be used. but i don't know if i'm even correct. and it's definitely not for this PR.