Add option to store the primal and derivative struct of all loop iterations

[michel2323]

@swilliamson7 mentioned the need to store intermediate derivatives to disk, which should be easy to add.

What about a store_intermediates option in the constructor like this?

revolve = Revolve{Heat}(steps, snaps; verbose=info; store_intermediates=true)

The user might want more granular control over what timesteps should be stored. We can add an interval or a list of timesteps. And a function to enable and disable it. Per default, the filenames would be named after the struct that is stored and the iteration counter.

store_intermediates(revolve, true)

[michel2323]

Added with #31 and #32 .