Add config attribute to store history of settings applied during network optimisation (and build?)

A solved network loaded from file has missing information pertaining, in particular, to settings used at the optimisation stage. This means it isn't entirely reproducible.

I would propose a Network.config attribute that stores this information. It could be generalised so that it has a optimize sub-key which stores the most recent set of args passed to the optimisation. This would leave open the option to extend it to storing applied settings for other network methods.

This config could be:

1. applied by default when running the optimize step, such that a user automatically runs with the same settings. They would then call something like n.config.clear() to reset it.
2. passed to the optimise function explicitly by the user, to achieve reproduction: n.optimize(**n.config.optimize).

This could be extended to other steps in the process of building and processing a network object, so that applied settings can be easily checked and/or can used as new baseline for defaults applied at different stages (if following the process defined in option (1) above).