General small fixes

[murphyqm]

While preparing figures and writing up the JOSS paper, I've spotted a few little bits to fix, saving them here for future reference:

• [x] In plotting module, allow max time and timestep to be passed in (currently hardcoded)

• [x] analysis.core_freezing requires the latent list as an argument, then takes this and uses len(latent) to calculate how long core is cooling for - this list isn't saved in json or arrays file, so this calc can't be done if loading in files. Makes more sense to add len(latent) to the json file than the full list

[murphyqm]

• [x] add separate package README
• [x] add tests for the saving/loading functions

[murphyqm]

• [x] Swap to tidier import function for tests and examples
• [x] Add "Start here!" example and figure out if they can be ordered by number

[murphyqm]

• Editing the example files to allow for the tidier import (without package installed) - this works much better with the examples gallery in the documentation (eg all the function calls now look the same). Going to go through the example notebooks, rewrite any extras (eg the reproducing Bryson one) in the format needed for sphinx gallery, add them to the docs - so both example folders are identical.