Feature request: update Ensemble's ancil

[hombit]

Current Ensemble.set_ancil rewrites the ancil completely and doesn't preserve what is already there. So this fails:

ens.set_ancil(dict(a=np.linspace(0, 1, n)))
ens.set_ancil(dict(b=np.linspace(1, 2, n)))
assert 'a' in ens.ancil

If I'd like to update it I could do something like this (in modern Python):

ens.set_ancil(ens.ancil | dict(b=np.linspace(1, 2, n)))

It would be fine to have update_ancil method of change the behavior of the current set_anciil.

NB. Actually, the current implementation of the ancil property also allows me to do this and change the underlying ens._encil in the way I want, is it intentional?

ens.ancil['b'] = "hello world"

I would suggest to change the implementation to produce a shallow copy of the attribute so users do not have an access to the original data (but still could accidentally change content of the underlying numpy arrays):

@property
def ancil(self):
    """Return the ancillary data dictionary"""
    return self._ancil.copy()

[aimalz]

Fixing this would also largely address LSSTDESC/rail_base#39, specifically the bits about copying over an ensemble's ancil when converting between parameterizations and the nonintuitive behavior of set_ancil and add_ancil, versus changing the value of the underlying _ancil attribute directly with =. Also, I'm happy to close that issue if folks agree it's appropriate to expand the scope of this one to add support for including ancil as a keyword argument upon instantiating a qp.Ensemble object, however, I'm probably not the best sole assignee due to unfamiliarity with the implications of the proposed design versus naive alternatives.

[eacharles]

So, the add_to_ancil function does update() with the addition of shape checking. I suppose it could be called update_ancil, but I think the behavior should be pretty clear from the name.

And the set_ancil function set's the entire dictionary, which is pretty standard behavior for a function called 'set_xxx'.