GDPuc 1.6.0

• Fixes #30, #32, #36
• Fix bug where countries without PPP data were not getting the US conversion factors.
• Fix dissagregtion of regions to ignore with missing conversion factors
• Make package compatible with madrat caching
• Add option to pass madrat regionmapping string to with_regions
• Drop dependency on "lifecycle" package.
• Improve documentation and add message when verbose is TRUE
• Introduce xCU as unit (local CU of any country x)
• Improve documentation
• Add arguments to specify non-default iso3c and year columns