Limit correlation to numeric scalars

[djlauk]

In its current display form, correlation (see #18) only works for numeric scalar data. I.e. it doesn't work for:

• data type string
• data type bool
• data shape waveform (= 1d array)
• data shape image (= 2d array)

The button should be disabled, unless there are exactly two channels selected, and both meet the above criteria.