New base task: base.context.from

[uwefladrich]

This is a follow-up to #67, implementing the possibility to load context variables from a YAML file, but also allowing to update the context from a previously defined dictionary. The implementation provides the feature from #67 but follows a different approach, hence, a new PR.

[mandresm]

Thanks, I have looked at the changes, and I don't have any comments. It's a really good feature to have. @JanStreffing, I tagged you here, in case you find this feature useful for you work on ECE4-F.