remove_primers: refactored

renamed primer.py to avoid clashes with obvious variable name
readme: notes about standards
fixed bad name change to usearch_
otu_caller: converted to config file values
map_barcodes: fixed erroneous usearch_python reference
fixed references to the config file

needed to specify not the filename but the directory
remove_primers: small comment
map_barcodes: major refactoring

still needs to be tested on a full data set. many barcodes appear to be
dropped.