symfit deprecationwarning

Switch to connectivity mapping

```
test/test_synthetic_data.py::TestSynthCell::test_measure_r
  C:\Users\jhs\Miniconda3\envs\py36_test_scipy\lib\site-packages\symfit\core\models.py:528: DeprecationWarning: `independent_vars` and `params` have been deprecated. Use `connectivity_mapping` instead.
    '`independent_vars` and `params` have been deprecated.'

```