Implement separate __eq__ methods

[nikfilippas]

As @rmjarvis and @marcpaterno correctly pointed out, we can save loads of time comparing CCLObjects if we compare parameters instead of strings. To that end, I extended some functionality that was already there to do parameter comparisons for most objects.

Just like the __repr_attrs__ class variable, which automatically builds a repr using a list of specified arguments, __eq_attrs__ now makes sure that the default __eq__ is overridden and that the individual attributes are compared one-by-one.
For example, in the Cosmology object, we only need to add this line in the beginning:

    __eq_attrs__ = ("_params_init_kwargs", "_config_init_kwargs", "_accuracy_params",)  # check these

    def __init__(...):
        ...

There are only 3 classes where this is not possible (because we have to check the C-level objects): Pk2D, Tk3D, Tracer. For these, I implemented full __eq__ methods as per the usual way.

Timings show that this way is indeed much faster (bld is the repr):

Note, however, that with the current implementation in master, the first computation of _repr might be slow, but every new call to __eq__ is of the order of nanoseconds (compare to microseconds with this PR proposal). It is a trade-off. So, if we compare objects sequentially (and discard the old one if they are not equal, like in an MCMC) this PR wins. But if we compare one object with several others, the currently implemented way wins.

[coveralls]

Pull Request Test Coverage Report for Build 4719168322

• 148 of 149 (99.33%) changed or added relevant lines in 13 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.09%) to 97.949%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pyccl/tracers.py	57	58	98.28%

Totals
Change from base Build 4577061095:	0.09%
Covered Lines:	5587
Relevant Lines:	5704

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💛 - Coveralls

[nikfilippas]

Just like with FancyRepr.disable() we can implement central control of __eq__ as well, which can disable __eq__ and use Python's default id-checking. This could potentially prove useful for cases where a developer makes a change but forgets to include a check in __eq__. Instead of overriding it, they could simply do ccl.EqControl.disable() or something like that.

[damonge]

Some comments. Will look at core.py next, then tests

[nikfilippas]

@damonge all done, back to you now and we can move on with the rest if you are happy with the changes.

[damonge]

Just one (maybe major) comment for core.py

[damonge]

Final comments on tests

[nikfilippas]

@damonge back to you

[damonge]

OK, we're done. Merge when the tests pass.