Use relative path for submodules

[jim-bcom]

Currently, committed URLs are HTTPS.

This requires the use of a token and frequent use of credentials.

SSH URLs allows a more convenient workflow.

We could convert all URLs to ssh instead, but then it becomes
inconsistent if someone clones SolAR in HTTPS (e.g. for read-only
purpose), and don't want to configure SSH keys.

And currently, if SolAR is cloned using SSH, then submitting changes to submodules may require tedious login/password authentication.

The solution can to use relative path, as described here:

https://www.damirscorner.com/blog/posts/20210423-ChangingUrlsOfGitSubmodules.html

This way, the submodule will be fetched with whatever method used
to clone SolAR.

This may lead to SSHerrors when git submodule foreach is used with
commands demanding a connection to the server (fetch, pull, ...).

A solution is to increase 'MaxStartups' in sshd config as described here:
https://docs.gitlab.com/ee/topics/git/troubleshooting_git.html#ssh_exchange_identification-error

(sshd needs to be restarted, or the machine rebooted)

[jim-bcom]

2 PRs are prepared:
#16
SolarFramework/SolARFramework#51