Issue 106: Unit testing for latent_individual

[athowes]

Description

This PR closes #106.

Notes:

• Include snapshot testing? Create issue for this?
• Added issue Checking formula input #118
• Added issue Using folders for unit and integration tests? #119
• Added issue Check data inputs to epidist_prepare #121
• Added issue Checking family argument #122

Checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have included the target issue or issues in the PR title in the for Issue(s) issue-numbers: PR title
• I have read the contribution guidelines.
• I have tested my changes locally.
• I have added or updated unit tests where necessary.
• I have updated the documentation if required.
• My code follows the established coding standards.
• I have added a news item linked to this PR.
• I have reviewed CI checks for this PR and addressed them as far as I am able.

[athowes]

Again failing I think due to #120.

[seabbs]

Really nice