add functions to read PMTCT and child ART inputs

[jeffeaton]

This PR adds several functions to read the PMTCT, Child ART, breastfeeding percentage, and abortion inputs from Spectrum Programme statistics input editor.

These are the functions and example usage:

pjnz <- system.file("pjnz/bwa2021_v6.13.pjnz", package = "leapfrog")
dp <- read_dp(pjnz)
dp_anc_testing <- dp_read_anc_testing(dp)
dp_pmtct <- dp_read_pmtct(dp)
dp_read_pmtct <- dp_read_pmtct_retained(dp)
dp_abortion <- dp_read_abortion(dp)
dp_notbreastfeeding <- dp_read_breastfeeding(dp)
dp_childart <- dp_read_childart(dp)
dp_childltfu <- dp_read_childltfu(dp)

## Can either pass PJNZ path or parsed "spectrum_dp" object

anc_testing1 <- dp_read_anc_testing(pjnz)
anc_testing2 <- dp_read_anc_testing(dp)
all.equal(anc_testing1, anc_testing2)

Things are returned as matrices with rows for indicators and columns for each year---mimicking the input editors in Spectrum.

Please review and let me know:

1. anything doesn't make sense,
2. any inputs you need that are missing,
3. any suggestions for better ways to format
4. anything that is wrong (of course)

A few further notes:

• I have not written tests for these functions yet.
• I have not written code that pulls them together into a set of inputs yet. Can discuss together strategy for that if useful.
• It is written for use with current version of Spectrum. We can add older tags for backward compatibility as needed; let me know.
• Let's Get this merged and pulled into your branches so that you can press ahead. But soon let's try to engage @r-ash to discuss plan for the inputs / outputs. Whether he would like to review the specio package for this or we should do something different.

[mwalte10]

Approving to merge into my branches, copying what is missing below in case it gets lost.

We are missing the following:

Transmission:

mtct transmission rates Distribution of new infections by CD4 Treatment:

ART initiation by age Disease progression:

HIV mortality without ART HIV mortality with ART Mortality rate ratios over time Disease progression rates