Canvas: check user input to avoid failing later?

[ceball]

Some examples of various errors during computation that would be much easier to debug if obviously invalid input values were rejected.

https://nbviewer.jupyter.org/github/ceball/snippets/blob/master/datashader-input-checking.ipynb

datashader input checking.ipynb.gz

[jbednar]

I think we should simply set it up to use param.Parameterized, which will automatically enforce boundaries like these and will provide users additional flexibility (e.g. to set parameters at the class level rather than having to use partial() or repeat arguments in multiple calls to the same code.