Add function to read and manipulate the population lookups

[Moohan]

The new function get_pop_est() works like the others in that you can do:

get_pop_est(level = "hb")

It will return the Health Board level populations (other levels can be got similarly), possibly it would be worth splitting this into get_hb_pop_est(), get_hscp_pop_est() etc?

There is other optional functionality:

• Group the data by age group: it uses phsmethods::create_age_groups() to create age groups so they can be customised using that function's parameters. The data will be returned aggregated to the age groups.
• The data can also be pivoted to return the various population breakdowns (age/age group and sex) as columns instead of rows. There are a number of options for this to chose different breakdowns.