Standardized constructor signature with dimensions as positional args

[philippjfr]

As suggested in #1938 this PR standardizes the constructors of Dimensioned objects such that kdims and vdims are the second and third argument respectively for Elements and kdims are the second argument for dimensioned containers (NdMapping, HoloMap, NdOverlay etc.). Currently Histogram and Annotations are the exceptions to this.

[jlstevens]

Are there backwards compatibility implications? Otherwise I support the idea.

[philippjfr]

Not as far as I know but I have yet to go through all the errors.

[jlstevens]

I suppose we haven't used them in a long time so they should be ok to remove now.

[philippjfr]

You would think that, turns out Columnar_Data still had a reference:

  3 error_hmap = hv.HoloMap({(i, j): hv.Image(j*np.random.randn(3, 3), bounds=extents)
  4                          for i, j in product(range(3), np.linspace(0, 1, 3))},
  5                         key_dimensions=['Observation', 'noise'])

[philippjfr]

Also addresses #1619

[philippjfr]

Okay, I've now gone through everything. There were two causes for errors:

1. In some very old tests we passed Image bounds as the second positional argument. Since bounds is now optional and is only used when passing a numpy array to Image I think moving away from that is sensible.
2. In some cases some internal code was passing NdMapping kdims as tuples, which used to be allowed but is now interpreted as a single dimension specification.

I think better exception messages will be sufficient to address both cases.

[jbednar]

Sounds reasonable, but be sure to link to this PR in the changelog section on backwards compatibility.

[jlstevens]

All sounds good to me!

I definitely wouldn't want bounds to be passed by position to Image anymore.

[philippjfr]

Tests now passing, I suppose I'll add some tests to ensure Elements follow this signature.

[philippjfr]

Ready to merge once test pass.

[jlstevens]

Something to mention in the next changelog/release notes. It will be good to get histogram working consistently with everything else.

[jlstevens]

Looks good to me. I made one comment about histogram but nothing that needs action. Merging.

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