Split sitrep into smaller unit packages

[zkamvar]

Currently sitrep does a few disparate things:

• provides inline styling of numbers with the fmt_count() family of functions
• tabulates linelist data with descriptive()
• tabulates survey data with tabulate_survey()
• plots age pyramids with plot_age_pyramid()
• generates fake data sets and surveys
• provides a parser for the MSF dictionaries
• creates age categories and groups them into a single variable
• provides helpers to calculate proportions (CFR, etc...)

It's safe to say that some of these can be broken into different packages because there are currently 34 packages that we import (which is not the number of packages that get installed).

When we know these templates are working, it would be a good idea to split some of these up into smaller packages that are viable stand-alone CRAN packages that sitrep can depend on (a la a kind of 'tidyverse' for epi packages).