Make calculator method to access the selected tax year
Add an error message when the select tax year doesn't contain a child
Add model spec for validation on tax year having children
Only raise an error on the child date, not tax year
Be valid where some children are inside the tax year
Fix trailing spaces in spec
Remove checks for nil values in child array

Whilst there might be a StartingChild with a nil start_date or
end_date, we aren't expecting any nil values present in the
array.
Merge remote-tracking branch 'origin/validate-tax-year-has-children'