Consolidate parameters

[MatthewKatzen]

Now that we are creating functions that take parameters from parent functions we need to make sure that all functions contain the relevant parameters and that the names are consistent.

Commonly used parameters:

1. fy.year (character) The financial year over which ... is to be calculated.
2. Date (Date vector or coercible to such) An alternative to \code{fy.year}
3. n_dependants (integer) Number of dependent children. By default, \code{0L}, so no children.
4. has_partner (logical) Is each individual married? By default, \code{FALSE}.