Feature: Update local YAMLs to tic templates and account for user changes

[pat-s]

This is a complex task. Below is sketch how this could work:

1. Every template comes with an ID so that we know which tic template we should look up
2. Every template has a part "TEMPLATE start/end" which will be strictly replaced with the upstream template
3. Then we enforce users to put local modifications to the templates in a section which is encapsulated by "USER CHANGES". During the update process, we look for this part and simply append it to the upstream template part
4. We check for possible duplicates and delete the duplicated parts from the TEMPLATE part since the user changes have higher preference
5. We tell the user which part we deleted from the TEMPLATE part due to the local changes which are stored in his YAML

Most often changes will be made for installing additional system libs. In this case, there will be no conflict at all because we do not define this part in the tic templates.
Other changes I see are mods to the env vars. This is a case that we should be able to catch in a descriptive way using the approach outlined above.

If users follow the principle to install any R deps only via tic.R, I am positive that this approach could work.
In the end, the user would only need to call tic::update_yaml() and everything will just work.

[pat-s]

Proposal

• Add a commented anchor which specifies the string that the respective use_*_yml() fun take, i.e. # linux-deploy.
• This anchor can then be found and trigger a call to use_*_yml(), updating the currently active template.

[pat-s]

Done in 0b9a80e.