MeasurementCsv: various enhancements

- Added values() and iter_values() methods. These return each row as a
  named tuple, with channel labels as the field names.
- __cmp__ has been made more generic by checking wether other has
  "value" attribute, rather than wether it is an instance of Measurment.
- MeasurementCsv no longer keeps an open handle to the file, and instead
  re-opens the file each time it needs it. This removes the need for
  managing the open handle, and alows parallel iterations over the
  values (each iteration will have it's own read handle into the files).

devlib/instrument/__init__.py

@@ -127,7 +127,7 @@ class Measurement(object):
         self.channel = channel
 
     def __cmp__(self, other):
-        if isinstance(other, Measurement):
+        if hasattr(other, 'value'):
             return cmp(self.value, other.value)
         else:
             return cmp(self.value, other)
@@ -147,26 +147,32 @@ class MeasurementsCsv(object):
         self.path = path
         self.channels = channels
         self.sample_rate_hz = sample_rate_hz
-        self._fh = open(path, 'rb')
         if self.channels is None:
             self._load_channels()
+        headings = [chan.label for chan in self.channels]
+        self.data_tuple = collections.namedtuple('csv_entry', headings)
 
     def measurements(self):
         return list(self.iter_measurements())
 
     def iter_measurements(self):
-        self._fh.seek(0)
-        reader = csv.reader(self._fh)
-        reader.next()  # headings
-        for row in reader:
+        for row in self._iter_rows():
             values = map(numeric, row)
             yield [Measurement(v, c) for (v, c) in zip(values, self.channels)]
 
+    def values(self):
+        return list(self.iter_values())
+
+    def iter_values(self):
+        for row in self._iter_rows():
+            values = map(numeric, row)
+            yield self.data_tuple(*values)
+
     def _load_channels(self):
-        self._fh.seek(0)
-        reader = csv.reader(self._fh)
-        header = reader.next()
-        self._fh.seek(0)
+        header = []
+        with open(self.path, 'rb') as fh:
+            reader = csv.reader(fh)
+            header = reader.next()
 
         self.channels = []
         for entry in header:
@@ -177,12 +183,23 @@ class MeasurementsCsv(object):
                     measure = mt
                     break
             else:
-                site = entry
-                measure = 'unknown'
+                if entry in MEASUREMENT_TYPES:
+                    site = None
+                    measure = entry
+                else:
+                    site = entry
+                    measure = 'unknown'
 
             chan = InstrumentChannel(site, measure)
             self.channels.append(chan)
 
+    def _iter_rows(self):
+        with open(self.path, 'rb') as fh:
+            reader = csv.reader(fh)
+            reader.next()  # headings
+            for row in reader:
+                yield row
+
 
 class InstrumentChannel(object):