Add calibration

This MR adds calibration uncertainty to Interferometer objects.

Currently, just the cubic spline model used elsewhere is implemented.

An example of usage has been added.

• Specifying the prior will be super boring so I should add a prior file for this.

.gitlab-ci.yml

@@ -29,6 +29,7 @@ exitcode-jessie:
     - coverage --version
     - coverage erase
     - coverage run --debug=trace --include=/opt/conda/lib/python3.6/site-packages/tupak* -a test/conversion_tests.py
+    - coverage run --include=/opt/conda/lib/python3.6/site-packages/tupak* -a test/calibration_tests.py
     - coverage run --include=/opt/conda/lib/python3.6/site-packages/tupak* -a test/detector_tests.py
     - coverage run --include=/opt/conda/lib/python3.6/site-packages/tupak* -a test/utils_tests.py
     - coverage run --include=/opt/conda/lib/python3.6/site-packages/tupak* -a test/prior_tests.py