Enable pickle compatibility for ligo.p_astro.Posterior class

This change implements the solution outlined in Issue #21, where dictionary keys and values stored on the Posterior class as attributes are instead stored as lists.

mentioned in merge request spiir!89 (merged)

mentioned in issue #21

Hi @lscsoft p-astro - I'd just like to bump this merge request so that SPIIR no longer has to maintain their own fork of p-astro.

Context

The LIGO reviewers for the SPIIR project recommended that we use this LIGO repository (as it is already reviewed etc.) rather than rewriting the same p-astro implementation from scratch, and so we'd very much like to continue using this repository for our current O4 needs (and clean up the unnecessary fork).

Changes

All we need is this two line change merged. I believe it is simple enough to verify from all uses of the self.args_all and self.keys_fixed attributes that downstream code should not be affected by this list function call we've used.

We've been using this code for almost 6 months now and we have run into no errors associated with this change.

self.args_all

For those familiar with Python, the only uses of self.args_all is either as an iterator ([arg.label for arg in self.args_all]), a call to len() (which accesses the object's __len__ method), and a cast to set. These three access patterns function exactly the same whether it is a list or dict_keys type.

self.keys_fixed

This attribute is used in exactly the same manner as self.args_all, and so the logic holds.

mentioned in merge request spiir-group/spiir!40 (merged)