New class 'profile', and friends `profile()`, `is_profile()`

This PR creates the class 'profile' and provides a the profiile() constructor. This also includes is_profile(), an a developer facing validator and constructor.

Advice from https://adv-r.hadley.nz/s3.html#s3-classes

The goal is to allow polymorphic behaviour downstrem.

TODO

[x] Link related issue/PR.
[x] Describe the goal of the PR. Avoid details that are clear in the diff.
[x] Mark the PR as draft.
[x] Include a unit test.
[x] Review your own PR in "Files changed".
[x] Ensure the PR branch is updated.
[x] Ensure the checks pass.
[x] Change the status from draft to ready.
[x] Polish the PR title and description.
[ ] Assign a reviewer.

EXCEPTIONS

[ ] Slide here any item that you intentionally choose to not do.

2DegreesInvesting / tiltIndicator

New class 'profile', and friends `profile()`, `is_profile()` #768