Output a plot of the learning curve by default

[Guillawme]

Describe the bug
This is not a bug but a suggestion for an improvement in user experience.

To Reproduce
Run cryodrgn analyze on any training result.
All result files are created, except for a plot of a learning curve.

Expected behavior
It would be helpful to also have a png file of the plot of the learning curve.

Additional context
At the moment, the learning curve plot is the only piece of result that is only found in the Jupyter notebook. This makes it difficult to access for less computationally inclined users (I have been helping such users, and the simplest interface between them and the results is inspecting png and mrc files). Looking at the learning curve to assess convergence is an important step, before spending more time looking at results. So, it would be very helpful to change cryodrgn analyze to make it generate a png file of this plot by default.