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

@@ -26,7 +26,11 @@ class PriorSet(dict):
         dict.__init__(self)
         if type(dictionary) is dict:
             self.update(dictionary)
-        elif filename or type(dictionary) is str:
+        elif type(dictionary) is str:
+            logger.debug('Argument "dictionary" is a string.'
+                         + ' Assuming it is intended as a file name.')
+            self.read_in_file(dictionary)
+        elif type(filename) is str:
             self.read_in_file(filename)
 
     def write_to_file(self, outdir, label):