Allow extending profiles and merging their contents.

[janstrohbeck]

This allows profiles to contain the following:

extends: <profile name>

The Context class then loads that base profile and merges the contents of the profiles (last value for a given key wins). When saving the context, the changes are saved in the correct files by keeping a lookup table which stores the file that each parameter was loaded from.

The use case I have for this is that we have many users, each with a separate profile. Most users just want to set a custom whitelist or blacklist and use default values from another profile for the cmake_args etc. With this PR, a lot of copied and pasted YAML code can be removed.

I do not know if this is something that is wanted in this project. If yes, I can add unit tests for the added functionality or refactor the changes if there is a nicer way to implement it (the key_origins feels a bit hacky).

Edit: extends might not be a good name as it is similar to extend_path. Also, I couldn't get the unit tests to work on my machine, so the CI build might fail.

[mikepurvis]

This looks like a reasonable and useful contribution. I don't think I have much quibble with the name— extends, parent, base, they're all basically the same thing. We do use "extend" to refer especially to the business of extending a workspace, but that's pretty separate from config profiles.

I'm happy to see tests come in a follow-on. Is there anything you want to add to this before it goes in?

[janstrohbeck]

We've used it at my organization for a couple of weeks and have not had any issues with it yet. It works even with 4 levels of extends :-)

So I think it can be merged as is. I probably won't have time to add tests in the next few weeks.

[mikepurvis]

Awesome, thanks for the contribution.