Popshare fix

[shahronak47]

Hi Andres, here's the fix for popshare argument.

Here are some pip calls to start with :

# From Tony's email
pip(country = "USA", year = 2022, popshare = 0.5, lkup = lkup)
pip(country = "USA", year = 2022, povline = 50, lkup = lkup)

# General tests
pip(country = "COL", year = 2022, popshare = 0.5, lkup = lkup)
pip(country = c("USA", "COL"),  year = 2022, popshare = 0.5, lkup = lkup)
pip(country = c("USA", "COL"),  year = 2022, povline= c(2.3, 4.5), lkup = lkup)

[randrescastaneda]

The changes in this PR have been included in #448