Handle datetimes and empty selections better for inspect operations

[philippjfr]

A number of fixes for inspect operations:

• Initialize with empty x/y coordinate
• Handle undefined x/y coordinates
• Allow datetime axis
• Allow axes with different dimensions, e.g. datetime and numeric (by normalizing space before computing distances)

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 7 lines in your changes missing coverage. Please review.

Project coverage is 88.50%. Comparing base (88996ed) to head (c1fac79).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
holoviews/operation/datashader.py	75.00%	6 Missing ⚠️
holoviews/tests/operation/test_datashader.py	94.44%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #6377    +/-   ##
========================================
  Coverage   88.50%   88.50%            
========================================
  Files         323      323            
  Lines       68281    68387   +106     
========================================
+ Hits        60429    60524    +95     
- Misses       7852     7863    +11     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[jbednar]

Thanks! Those changes look good to me, though I'm not an expert in datetime dtypes.

[hoxbro]

Some nit comments.