Save project custom file paths as relative

[alexhroom]

Currently, custom files use absolute paths, which means that the project.json files are not portable. They should be changed so that the custom file paths saved in the project.json file are relative to the file save location, e.g. using Path.relative_to

[StephenNneji]

The current save methods expect path to be provided, I think it would be nice to have the path be optional and no path will save to the current directory then the user can write problem.save() instead of problem.save("")

[StephenNneji]

Also why not save(path = "project.json")? I am not sure there is much benefit to splitting the path and name

[alexhroom]

@StephenNneji the idea was to eliminate some ambiguity: if a user calls project.save(path="~/Documents/my_project"), do they want to save a file called my_project.json in Documents or do they want to save to the directory ~/Documents/my_project/? although looking at some Python standard libraries e.g. pickle or json it seems the idiom is that we assume the path is the path to the file object, and the MATLAB save function also behaves this way, so I guess we can combine them and be opinionated there!