make a single assert_valid_posit_id function for accessors

[AdrienLeGuillou]

the posit_ids checks for accessors where made prior to UID / PID implementation.
Thus they do not check for NAs in posit IDs (created from get_unique_ids with deactivated nodes

This function could implement all checks for posit_ids and error if any of them fails (with a comprehensive message)

tests:

• any NA
• any PID > population size

Open question:
should we still allow posit_ids to be a logical vector? This was to mimic the vector subsetting but I am not sure it is used and it makes the code heavier