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.

tupak/core/prior.py

@@ -39,7 +39,7 @@ class PriorSet(dict):
             Output file naming scheme
         """
 
-        prior_file = os.path.join(outdir, "{}_prior.txt".format(label))
+        prior_file = os.path.join(outdir, "{}.prior".format(label))
         logger.debug("Writing priors to {}".format(prior_file))
         with open(prior_file, "w") as outfile:
             for key in self.keys():