add compute() to dynamically show/hide sidebar computation panel

[sc1f]

Because computed columns is not yet implemented in the Python API, make sure the viewer does not render the computation sidebar when Table.compute() returns false.

[codecov-io]

Codecov Report

Merging #748 into master will decrease coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #748      +/-   ##
==========================================
- Coverage   91.82%   91.69%   -0.13%     
==========================================
  Files          49       49              
  Lines        3093     3095       +2     
==========================================
- Hits         2840     2838       -2     
- Misses        253      257       +4

Impacted Files	Coverage Δ
perspective/table/_date_validator.py	86.84% <0%> (-7.9%)	⬇️
perspective/table/table.py	94% <0%> (-0.9%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 249b10a...4efea02. Read the comment docs.

[texodus]

Looks good, thanks for the PR!