make 'fname' a parameter to 'save' and 'load' only

This simplifies the API by making sure that the filenames are only provided in one place (the calls to save and load).

Closes #122 (closed)

adaptive/learner/balancing_learner.py

 # -*- coding: utf-8 -*-
-from collections import defaultdict
+from collections import defaultdict, Iterable
 from contextlib import suppress
 from functools import partial
 from operator import itemgetter
@@ -323,8 +323,9 @@ class BalancingLearner(BaseLearner):
 
         Parameters
         ----------
-        fname: callable
-            Given a learner, returns a filename into which to save the data
+        fname: callable or sequence of strings
+            Given a learner, returns a filename into which to save the data.
+            Or a list (or iterable) with filenames.
         compress : bool, default True
             Compress the data upon saving using `gzip`. When saving
             using compression, one must load it with compression too.
@@ -347,8 +348,12 @@ class BalancingLearner(BaseLearner):
         >>> # Then save
         >>> learner.save(combo_fname)  # use 'load' in the same way
         """
-        for l in self.learners:
-            l.save(fname(l), compress=compress)
+        if isinstance(fname, Iterable):
+            for l, _fname in zip(fname, self.learners):
+                l.save(_fname, compress=compress)
+        else:
+            for l in self.learners:
+                l.save(fname(l), compress=compress)
 
     def load(self, fname, compress=True):
         """Load the data of the child learners from pickle files
@@ -356,8 +361,9 @@ class BalancingLearner(BaseLearner):
 
         Parameters
         ----------
-        fname: callable
-            Given a learner, returns a filename into which to save the data
+        fname: callable or sequence of strings
+            Given a learner, returns a filename from which to load the data.
+            Or a list (or iterable) with filenames.
         compress : bool, default True
             If the data is compressed when saved, one must load it
             with compression too.
@@ -366,8 +372,12 @@ class BalancingLearner(BaseLearner):
         -------
         See the example in the `BalancingLearner.save` doc-string.
         """
-        for l in self.learners:
-            l.load(fname(l), compress=compress)
+        if isinstance(fname, Iterable):
+            for l, _fname in zip(fname, self.learners):
+                l.load(_fname, compress=compress)
+        else:
+            for l in self.learners:
+                l.load(fname(l), compress=compress)
 
     def _get_data(self):
         return [l._get_data() for l in learner.learners]