Introduce a common parent class for NumericalColumn and DecimalColumn

[vyasr]

Adds a common parent class for NumericalColumn and DecimalColumn that implements the common operations (mainly reductions and scans). As part of enabling this, the _copy_type_metadata method was refactored to have different column types override it to perform column-specific tasks (necessary because DecimalColumn has some special handling), which should also provide some marginal performance improvements for methods that copy metadata.

This PR (and some to follow) are designed to help simplify the ongoing Array refactoring.

[galipremsagar]

@vyasr Looks like there are some test failures in test_transform.py that are linked to this refactor, could you take a look at it?

[vyasr]

Yes sorry I noticed this failure but I've been focusing on #8214 for the past few days. I should hit a good stopping point this afternoon to come back and wrap up my open Python PRs including this one.

[kkraus14]

Marking this as breaking since we don't technically hide the Column class even though we don't intend for it to be part of our public API.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (branch-21.06@cbbcba7). Click here to learn what that means.
The diff coverage is n/a.

@@               Coverage Diff               @@
##             branch-21.06    #8278   +/-   ##
===============================================
  Coverage                ?   82.88%           
===============================================
  Files                   ?      106           
  Lines                   ?    17844           
  Branches                ?        0           
===============================================
  Hits                    ?    14790           
  Misses                  ?     3054           
  Partials                ?        0           

---

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 cbbcba7...442fcda. Read the comment docs.

[kkraus14]

@gpucibot merge