Define how fitter node can tell if an analysis is possible

[wpfff]

For example: dimension of a fit generally: what type of data does a Fit even accept to work correctly?

Idea: have a function per analysis that checks for validity?

Eventually: How should plottr behave when data is not valid? For example: if we have 2d data, but 1d fit: should we automatically 'batch' fit? (not most urgent)