ENH: compute kcorrect remaining stellar mass

[ntessore]

Description

Adds the stellar_mass_remain() function to the kcorrect template class which computes total surviving stellar mass

Checklist

• [x] Follow the Contributor Guidelines
• [x] Write unit tests
• [x] Write documentation strings
• [x] Assign someone from your working team to review this pull request
• [x] Assign someone from the infrastructure team to review this pull request

[itrharrison]

We are happy to merge this but I will write a test first!