Improvements to checkpointing to align it with bilby!746

See bilby!746 (merged)

1. Pickle the entire sampler
2. Save every check_point_deltaT seconds
3. Add stats and run plots at each iteration

tests/parser_test.py

@@ -46,7 +46,7 @@ class ParserTest(unittest.TestCase):
             mem_per_cpu="1000",
             min_eff=10,
             minimum_frequency="20",
-            n_check_point=100000,
+            n_check_point=100,
             n_effective=np.inf,
             n_simulation=0,
             n_parallel=4,