Authenticates a TissueMAPS user.

        Parameters
        ----------
        username: str
            name
        password: str
            password

def _login(self, username, password):
        '''Authenticates a TissueMAPS user.

        Parameters
        ----------
        username: str
            name
        password: str
            password
        '''
        logger.debug('login in as user "%s"' % username)
        url = self._build_url('/auth')
        payload = {'username': username, 'password': password}
        res = self._session.post(url, json=payload)
        res.raise_for_status()
        self._access_token = res.json()['access_token']
        self._session.headers.update(
            {'Authorization': 'JWT %s' % self._access_token}
        )