Improve output text to screen

[jotaylor]

It could be cleaned up a bit, including

• having fewer iteration printouts
• getting rid of some useless text

Currently it looks like:

Mean acceptance fraction: 0.332
Saved pickle file 20190522_1327/model_20190522_1327.pickle.gz
	Saved model_20190522_1327_triangle.png
Plotting the MCMC chains.
	Saved model_20190522_1327_chain.png
Plotting the model posterior PDFs.
	Saved model_20190522_1327_posterior.png
Saved 20190522_1327/model_20190522_1327_posterior.pdf
executed in 0:00:12.307435