Spectrum refactoring, Spectrum2DCollection

[ajjackson]

As part of a major refactor of Abins, we would like to use a Spectrum2DCollection object which behaves similarly to Spectrum1DCollection. This allows the code to largely treat 1D and 2D data interchangably, simplifying the logic and providing access to select/groupby "table-like" operations on the data.

This is implemented by factoring out the "collection"-specific methods to a "mixin" class. This gives a reasonable class heirarchy in which Spectrum1D, Spectrum2D, Spectrum1DCollection and Spectrum2DCollection are all children of Spectrum but not of each other.

In the process, we also found some limitations and inefficiencies in the existing Spectrum objects:

• iteration over Spectrum1DCollection is very slow due to redundant copying, checks and unit conversion
  • to allow cheaper iterations over metadata without creating new objects, this PR adds an iter_metadata method that iterates over metadata lines, merging the top-level and "line-data" parts on the fly. This is also a good building block to simplify some of the other metadata-handling methods.
• select() and group_by() do not properly handle metadata that is common to all rows (and so in the top-level of the metadata dict). This is made more robust in reimplementations of those methods.

[github-actions[bot]]

Test Results

    22 files  ± 0      22 suites  ±0   30m 6s :stopwatch: +50s  1 054 tests + 6   1 048 :white_check_mark: + 6   6 :zzz: ±0  0 :x: ±0  10 480 runs  +60  10 417 :white_check_mark: +60  63 :zzz: ±0  0 :x: ±0 

Results for commit 0f7ec817. ± Comparison against base commit 82688812.

:recycle: This comment has been updated with latest results.

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Codecov Report

Attention: Patch coverage is 87.12871% with 13 lines in your changes missing coverage. Please review.

Project coverage is 95.47%. Comparing base (7608b14) to head (a50fc98). Report is 20 commits behind head on master.

Files	Patch %	Lines
euphonic/spectra.py	87.12%	11 Missing and 2 partials :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #305 +/- ## ========================================== - Coverage 95.85% 95.47% -0.39% ========================================== Files 28 28 Lines 3982 4021 +39 Branches 803 816 +13 ========================================== + Hits 3817 3839 +22 - Misses 97 109 +12 - Partials 68 73 +5 ```

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