Enhancements for Pk2D, Tk3D, Tracer.

[nikfilippas]

This PR adds a number of features to the Pk2D object.

1. Pk2Ds which are initialized with empty=True never contain attributes extrap_order_hik and extrap_order_lok. This is a bug, and it can be fixed by making both attributes properties of the object.
2. has_psp has to manually be set every time an empty object is populated with a psp. It makes more sense for has_psp to also be a property that peeks into psp to see if it's there.
3. Currently, apply_halofit is a class method. It makes more sense for it to be called as an instance method, as it operates using the implicit first argument (which is now passed last, explicitly). Instead of ccl.Pk2D.apply_halofit(cosmo, pk), we can now do pk.apply_halofit(cosmo). I implemented a descriptor that allows it to work both as a class method and as an instance method, with the ultimate goal to deprecate the class method usage in the future.
4. Simplify call to pk models: ccl.Pk2D.pk_from_model --> ccl.Pk2D.from_model.
5. eval and eval_dlogpk_dlogk are very similar, so I concatenated them, shortening the code base, but keeping both definitions.
6. Option to call eval without cosmo. It will fail if a value outside the scale factor range of psp is requested. This was built in the C code already. For some reason, it wasn't ported to Python.
7. Implement __call__ for Pk2D and Tk3D. Instead of np.array([pk.eval(k_arr, a) for a in a_arr]), we can now do pk(k_arr, a_arr).
8. Implement copy. Initialization is very fast, so copy will enable us to operate on copies of Pk2D objects.
9. Does my pk (or tk or tr) have data? bool(pk)!
10. Is pk1 within the k- and a- boundaries of another pk2? pk1 in pk2! (Useful when operating with different supports.)
11. Extended binary operations: pk1 - pk2, pk1 / pk2, pk += 0.05, pk *= 1.05, pk -= 0.05, pk /= 0.95, pk **= 2.

[coveralls]

Pull Request Test Coverage Report for Build 2796012781

• 121 of 121 (100.0%) changed or added relevant lines in 4 files are covered.
• 16 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.2%) to 97.502%

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Files with Coverage Reduction	New Missed Lines	%
pyccl/pyutils.py	16	90.94%

Totals
Change from base Build 2781888489:	0.2%
Covered Lines:	4840
Relevant Lines:	4964

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💛 - Coveralls

[damonge]

Alright, a nice one!
Lots of comments, but nothing major.

[nikfilippas]

@damonge back to you.

[damonge]

Alright @nikfilippas , almost there

[nikfilippas]

@damonge I think I addressed your final comments, have a look and let me know if you have any more questions/suggestions.

[damonge]

LGTM!