BUGFIX: fix act-walk caching

@serguei.ossokine pointed out that the act-walk is not working with parallelization, the reason for the issue is that the object gets pickled at each call to the sampler and so instance-level attributes aren't being preserved properly.

To fix this, I made the _cache a class-level attribute. I also made sure that the cache is wiped at the beginning of sampling in case people run multiple analyses with this method using a single script/session in a jupyter notebook.

I removed the use of premade instances of the walking methods as it wasn't achieving anything.

EDIT: there was an equivalent problem with the rwalk method that had a similar solution

bilby/core/sampler/dynesty.py

@@ -450,32 +450,32 @@ class Dynesty(NestedSampler):
                 f"An average of {2 * self.nact} steps will be accepted up to chain length "
                 f"{self.maxmcmc}."
             )
-            from .dynesty_utils import sample_rwalk_bilby
+            from .dynesty_utils import AcceptanceTrackingRWalk
 
             if self.kwargs["walks"] > self.maxmcmc:
                 raise DynestySetupError("You have maxmcmc < walks (minimum mcmc)")
             if self.nact < 1:
                 raise DynestySetupError("Unable to run with nact < 1")
-            dynesty.nestedsamplers._SAMPLING["rwalk"] = sample_rwalk_bilby
+            AcceptanceTrackingRWalk.old_act = None
+            dynesty.nestedsamplers._SAMPLING["rwalk"] = AcceptanceTrackingRWalk()
         elif sample == "acceptance-walk":
             logger.info(
                 "Using the bilby-implemented rwalk sampling with an average of "
                 f"{self.naccept} accepted steps per MCMC and maximum length {self.maxmcmc}"
             )
-            from .dynesty_utils import fixed_length_rwalk_bilby
+            from .dynesty_utils import FixedRWalk
 
-            dynesty.nestedsamplers._SAMPLING[
-                "acceptance-walk"
-            ] = fixed_length_rwalk_bilby
+            dynesty.nestedsamplers._SAMPLING["acceptance-walk"] = FixedRWalk()
         elif sample == "act-walk":
             logger.info(
                 "Using the bilby-implemented rwalk sampling tracking the "
                 f"autocorrelation function and thinning by "
                 f"{self.nact} with maximum length {self.nact * self.maxmcmc}"
             )
-            from .dynesty_utils import bilby_act_rwalk
+            from .dynesty_utils import ACTTrackingRWalk
 
-            dynesty.nestedsamplers._SAMPLING["act-walk"] = bilby_act_rwalk
+            ACTTrackingRWalk._cache = list()
+            dynesty.nestedsamplers._SAMPLING["act-walk"] = ACTTrackingRWalk()
         elif sample == "rwalk_dynesty":
             sample = sample.strip("_dynesty")
             self.kwargs["sample"] = sample