feat: save stats to IterationHistory

[Solosneros]

I added IterationHistory which is a extended dict that can be used to record the statistics of VBMC. The general logic (which might have been occasionally violated in MATLAB) is that optimState (now vbmc object) would store the current value in the iteration, and stats (now IterationHistory) would record the history per iteration.

The main differences to a normal dict are:

• IterationHistory takes care of expanding the respective arrays in every iteration
• Values are deepcopied so that stats of one iteration are not modified later
• iteration_history.record_iteration() is a convenience method to record the stats of a whole iteration.

Please note that some implementation and documentation depends on the further implementation of VBMC and other objects and must be enhanced in a later commit.

Test-coverage of IterationHistory: 100%, I think I tested every major case.

[Solosneros]

@lacerbi Please merge the #10 before this one. I might have to merge the main branch into this one afterwards. :) (There are a few overlaps between #12 and #10)

[Solosneros]

Record iteration from dict instead of many single parameters and improve robustness by checking for the existence of keys that must be specified on initialization of the object.