get_table() should output a single version/year

[ernestguevarra]

Is your feature request related to a problem? Please describe.
Currently, get_tables() may output more than one lookup table for a country as there is no way to specify which PPI table version/year to output

Describe the solution you'd like
add a year argument to get_table() which will by default be set to NULL. If NULL, output will be the latest PPI table version. If specified, check if that year is available and then return that year if it is. If not, return NULL and give a warning.

Describe alternatives you've considered
Status quo will just need to be filtered by user to the version they need.

Additional context
NA