code: def add_sample(self, samp_name, site_name=None, er_data=None, pmag_data=None):
          
          if site_name:
              site = self.find_by_name(site_name, self.sites)
              if not site:
                  print("""-W- {} is not a currently existing site.
  Creating a new site named: {} """.format(site_name, site_name))
                  site = self.add_site(site_name)
          else:
              site = None
          sample = Sample(samp_name, site, self.data_model, er_data, pmag_data)
          self.samples.append(sample)
          if site:
              site.samples.append(sample)
          return sample
docs: Create a Sample object and add it to self.samples.
          If a site name is provided, add the sample to site.samples as well.