using composite primary keys for cals

[MartinHinz]

Even though #331 should be resolved for now with the merge of caldates, I believe a long-term and sustainable solution would be to implement composite primary keys for the relationship between c14s and cals via bp and std. Along those lines, it might also be worth reconsidering the naming strategy, to have consistency regarding bp to c14_age and std to c14_error.

Link provided by Joe: https://edgeguides.rubyonrails.org/7_1_release_notes.html#composite-primary-keys