function to retrieve vaccination data fixes #35

This adds a function to retrieve.R and does not modify any other code.

The availability of vaccine data is limited at this point, and I'd like to revisit this at some point later.

The data at https://covid.cdc.gov/covid-data-tracker/#vaccinations seems to only be available in a Tableau (?) download data box with no direct URL link easily determined from the site source
youyanggu mentioned his aggregated data on the mailing list (https://github.com/signaturescience/focustools/issues/35#issuecomment-769852747) - it appears he's using some of the data from CDC but unclear how he's getting it or how it's being aggregated. If we could find the source for this this may be a better choice.
Ajitesh emailed the mailing list (https://github.com/signaturescience/focustools/issues/35#issuecomment-768166691) with links to his vaccine forecasts. It appears he's using a combination of Our World in Data and Centers for Civic Impact.

This get_vax.R function is pulling from Our World in Data, which is CC-BY, and summarizing to epiyear and epiweek. We currently only have a few weeks worth of data, but we have it on US, States, others (see ?get_vax). Attribution is given in the function documentation.

Some stats on US, Tx, VA:

location	epiyear	epiweek	total_distributed	total_vaccinations	proportion_vaccinations_given	people_vaccinated	people_fully_vaccinated	population	total_vaccinations_percap
US	2021	2	118865525	43033771	0.36	35700954	3413012	328728466	0.13
US	2021	3	186415175	89425192	0.48	76552939	12364322	328728466	0.27
US	2021	4	321990525	176432771	0.55	146965706	28149837	328728466	0.54
US	2021	5	49933250	31123299	0.62	25201143	5657142	328728466	0.09
48	2021	2	8144550	4093369	0.50	3591019	372942	28995881	0.14
48	2021	3	13366775	7792936	0.58	6730994	1055253	28995881	0.27
48	2021	4	23822000	14127105	0.59	11721184	2394578	28995881	0.49
48	2021	5	3659550	2408879	0.66	1928227	478812	28995881	0.08
51	2021	2	3348500	870513	0.26	407650	28164	8535519	0.10
51	2021	3	4945500	1863097	0.38	1644667	208269	8535519	0.22
51	2021	4	8081850	4337577	0.54	3630202	563405	8535519	0.51
51	2021	5	1232350	833221	0.68	687883	118612	8535519	0.10

closes #35

errrr maybe im missing something here ... but im not understanding this output. @stephenturner i need you to walk me through this before i merge.

are these numbers cumulative? or "incident" ??

take a look at 2021 epiweek 4 (row 3 above) ... "people_vaccinated" in the US is 146965706 ... even if that's cumulative ... that's 44% of the US population!

like i said i might be misinterpreting the columns. but if so that's an issue we should resolve too. maybe by simplifying the output of get_vax() to look more like get_cases() / get_deaths():

vax <- get_vax()
vax %>% 
  mutate(partial = people_vaccinated - people_fully_vaccinated) %>%
  rename(full = people_fully_vaccinated) %>%
  select(location, epiyear, epiweek, ipartial = partial, ifull = full) %>%
  group_by(location) %>%
  mutate(cpartial = cumsum(ipartial),
         cfull = cumsum(ifull)) %>%
  head(10) %>%
  yawp::more() %>%
  knitr::kable()

location	epiyear	epiweek	ipartial	ifull	cpartial	cfull
US	2021	2	32287942	3413012	32287942	3413012
US	2021	3	64188617	12364322	96476559	15777334
US	2021	4	118815869	28149837	215292428	43927171
US	2021	5	19544001	5657142	234836429	49584313
01	2021	2	283086	30003	283086	30003
01	2021	3	640224	107909	923310	137912
01	2021	4	1417602	259488	2340912	397400
01	2021	5	242970	55331	2583882	452731
02	2021	2	101109	20972	101109	20972
02	2021	3	248709	63361	349818	84333
.	.	.	.	.	.	.
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^ those numbers are off. but might be worth using that format? idk.

signaturescience / focustools

function to retrieve vaccination data fixes #35 #41