Add abstract types for single- and multi-target MLJ Regressors.

[atharvas]

Right now, SRRegressor and MultitargetSRRegressor subtype AbstractSRRegresor. However, much of the src/MLJInterface.jl code dispatches on either SRRegressor or MultitargetSRRegressor. This creates a bottleneck for defining custom MLJInterfaces and forces redefinition of much of src/MLJInterface.jl. The commit addresses this by introducing two new types, SingletargetAbstractSRRegressor and MultitargetAbstractSRRegressor, and changes the logic in src/MLJInterface.jl to dispatch on these types instead.

Note: This approach is kind of janky as it solves the reuse problem using multiple inheritance (which might cause problems of its own if abused). This can be trimmed further / I'm open to rethinking this.

[github-actions]

Benchmark Results

	master	e6a502d...	master/e6a502d8c102da...
search/multithreading	17.7 ± 0.82 s	17.3 ± 2.1 s	1.02
search/serial	28.9 ± 1.1 s	30 ± 1.2 s	0.965
utils/best_of_sample	1.63 ± 0.42 μs	1.76 ± 0.72 μs	0.926
utils/check_constraints_x10	11.4 ± 3 μs	11.3 ± 2.9 μs	1.02
utils/compute_complexity_x10/Float64	2.11 ± 0.09 μs	2.12 ± 0.1 μs	0.995
utils/compute_complexity_x10/Int64	2.11 ± 1.6 μs	2.11 ± 1.5 μs	1
utils/compute_complexity_x10/nothing	1.53 ± 0.08 μs	1.49 ± 0.1 μs	1.03
utils/insert_random_op_x10	5.71 ± 1.8 μs	5.81 ± 1.8 μs	0.984
utils/next_generation_x100	0.35 ± 0.092 ms	0.359 ± 0.098 ms	0.977
utils/optimize_constants_x10	0.0338 ± 0.0079 s	0.0356 ± 0.0086 s	0.951
utils/randomly_rotate_tree_x10	5.41 ± 0.7 μs	5.43 ± 0.73 μs	0.996
time_to_load	1.89 ± 0.027 s	1.94 ± 0.0042 s	0.975

Benchmark Plots

A plot of the benchmark results have been uploaded as an artifact to the workflow run for this PR.
Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).

[MilesCranmer]

Sounds good to me. One minor thing is it should be AbstractMultitargetSRRegressor per usual Julia style of Abstract going at the start of the type name. So perhaps AbstractSRRegressor should be named to AbstractSymbolicRegressor or something, so it sounds more "general"?

[coveralls]

Pull Request Test Coverage Report for Build 12680133314

Details

• 14 of 15 (93.33%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.786%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/MLJInterface.jl	13	14	92.86%

Totals
Change from base Build 12680128577:	0.0%
Covered Lines:	3163
Relevant Lines:	3337

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💛 - Coveralls