Standardise internal pipeline functions

[LiamDBailey]

Current internal functions like calc_age and calc_clutchtype have different behaviours, one takes and returns a data frame the other takes and returns vector(s). We need to create a standard so that all internal functions behave in a predictable way.

• All internal functions should take a data frame and return a data frame. This allows them to be used easily in the pipeline. Note This will mean that they expect the data frame in a particular format. We will assume that the input data frame is in the standard format.

We can add our standards for internal functions to the document created with #127

[LiamDBailey]

@zuzulaz could you also create some tests for these functions once they're standardised?