Fix CRAN submission feeback

[tonyfujs]

• Please always write package names, software names and API (application programming interface) names in single quotes in title and description. e.g: --> 'PIP'. Please note that package names are case sensitive.

• Please add a web reference for the API in the form https:..... to the description of the DESCRIPTION file with no space after 'https:' and angle brackets for auto-linking.

• Please add \value to .Rd files regarding exported methods and explain the functions results in the documentation.

• Please write about the structure of the output (class) and also what the output means. (If a function does not return a value, please document that too, e.g. \value{No return value, called for side effects} or similar)

• Missing Rd-tags:
  get_countries.Rd: \value
  get_cpi.Rd: \value
  get_dictionary.Rd: \value
  get_gdp.Rd: \value
  get_hfce.Rd: \value
  get_incgrp_coverage.Rd: \value
  get_interpolated_means.Rd: \value
  get_pop.Rd: \value
  get_pop_region.Rd: \value
  get_ppp.Rd: \value
  get_region_coverage.Rd: \value
  get_regions.Rd: \value
  get_survey_means.Rd: \value

• Please do not modify the global environment (e.g. by using <<-) in your functions. This is not allowed by the CRAN policies. -> R/zzz.R. This last ticket item is trickier as it modifies the logic of how we handle caching. I created a dedicated ticket for it: issue Don't modify global environment #61