Fix pairplots_posteriors

[cranedroesch]

currently gives following error when uncommented (towards bottom of _01_GOF_sims):

	Traceback (most recent call last):
	  File "_01_GOF_sims.py", line 740, in <module>
	    main()
	  File "_01_GOF_sims.py", line 728, in main
	    plt_pairplot_posteriors(toplot, figdir, prefix=prefix)
	  File "/Users/crandrew/projects/chime_sims/_02_munge_chains.py", line 181, in plt_pairplot_posteriors
	    grid = sns.PairGrid(data=df.sample(n))
	  File "/opt/anaconda3/lib/python3.7/site-packages/pandas/core/generic.py", line 5061, in sample
	    locs = rs.choice(axis_length, size=n, replace=replace, p=weights)
	  File "mtrand.pyx", line 946, in numpy.random.mtrand.RandomState.choice
	ValueError: Cannot take a larger sample than population when 'replace=False'