make mcmc more efficient! and hopefully better!

This modifies the mcmc sampler in parallel mode. Before, we were appending the initial live point to the list of potential samples. Once we've take a certain number of mcmc walks, we randomly sample from this list to choose a proposal point. The issue is that the list mainly contains samples that are guaranteed to be rejected, and the probability of accepting a good point is greatly reduced. This branch removes that issue, making sampling more efficient and probably more effective at sampling the corners of the posterior.

mentioned in merge request bilby!730 (closed)

added 14 commits

• 6cc5af25...26787293 - 12 commits from branch master
• 7f06415d - Merge branch 'master' into make_mcmc_better_and_more_efficient
• adb5040f - Remove n-parallel

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added 2 commits

• e39c366b - Clean up and add messages
• 0b39a828 - add in check for i

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merged

mentioned in commit 359b3152