code: def to_pn(self, sub_letter=None):
          
          string = str(self)
          if '.' not in string:
              return string
  
          # take care of the case of when there is no scaling unit
          if not string[-1].isalpha():
              if sub_letter is not None:
                  return string.replace('.', sub_letter)
  
              return string
  
          letter = string[-1]
          return string.replace('.', letter)[:-1]
docs: Returns the part number equivalent.  For instance,
          a '1k' would still be '1k', but a
          '1.2k' would, instead, be a '1k2'
          :return: