Improve dtype inspection

[sjperkins]

No description provided.

[sjperkins]

@o-smirnov This resolves the test case issues in holoviz#926 for me, locally.

[o-smirnov]

So do you want me to test this with shadems before you submit it to the main repo.

[sjperkins]

This makes the datashader test suite pass locally for me.

I also ran:

$ shadems ~/data/AF0236_spw01.ms -x real -y imag 
$ shadems ~/data/AF0236_spw01.ms -x real -y imag -c FIELD_ID --count-cat

and they ran to completion, but I'm no expert in reading the tea leaves in the plot.

I wouldn't expect any breakage in the underlying functionality: the change exists to fix metadata handling. Maybe run it once on a test case you know well?

[sjperkins]

So do you want me to test this with shadems before you submit it to the main repo.

I should have read this more carefully. Can you merge this into your treered-ds11 branch? I guess this will automatically update the PR and cause the datashader test suite to pass.

[sjperkins]

It may be better for me to create the PR from my datashader fork as I'll be the one responding to their PR review comments.