Operation: refactor to use inheritance

[dbaston]

The following changes are made to Operation:

• All Operations now assign results to a single column. This affects only the "frac" stat, which now returns an array of fractions instead of magically populating columns like "frac_1", "frac_2", etc. This simplifies the implementation of several classes and removes table-reshaping functionality that is better handled elsewhere.
• All information about a given Operation is now consolidated in its implementation, instead of scattered among string comparisons in multiple "if" blocks.
• The validity of an Operation is now checked when it is instantiated, rather than when results are first written.

[codecov[bot]]

Codecov Report

Attention: 22 lines in your changes are missing coverage. Please review.

Comparison is base (02e7326) 86.32% compared to head (da74c45) 85.66%.

Files	Patch %	Lines
src/gdal_feature.h	55.55%	8 Missing :warning:
src/feature.cpp	86.84%	5 Missing :warning:
src/operation.cpp	97.09%	5 Missing :warning:
src/gdal_writer.cpp	87.50%	3 Missing :warning:
src/deferred_gdal_writer.cpp	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #76 +/- ## ========================================== - Coverage 86.32% 85.66% -0.66% ========================================== Files 54 54 Lines 2735 2805 +70 ========================================== + Hits 2361 2403 +42 - Misses 374 402 +28 ```

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