instrument: Add channel discovery to MeasurementCsv

- Add "unknown" measurement type to be used when the type of measurement
  for a CSV column cannot be established.
- Make channels argument optional for MeasurementCsv.
- If channels for a MeasurementCsv have not been specified, attempt to
  discover them from the CSV header. This will  check if each entry in
  the header ends with the name of a known MeasurementType prefixed with
  a "_"; if so, it will assume the reset of the entry is the site.
  Otherwise, the entire entry will be assumed to be the site and
  "unknown" MeasurementType will be used.

devlib/instrument/__init__.py

@@ -72,6 +72,7 @@ class MeasurementType(object):
 
 # Standard measures
 _measurement_types = [
+    MeasurementType('unknown', None),
     MeasurementType('time', 'seconds',
         conversions={
             'time_us': lambda x: x * 1000,
@@ -132,10 +133,12 @@ class Measurement(object):
 
 class MeasurementsCsv(object):
 
-    def __init__(self, path, channels):
+    def __init__(self, path, channels=None):
         self.path = path
         self.channels = channels
         self._fh = open(path, 'rb')
+        if self.channels is None:
+            self._load_channels()
 
     def measurements(self):
         return list(self.itermeasurements())
@@ -148,6 +151,29 @@ class MeasurementsCsv(object):
             values = map(numeric, row)
             yield [Measurement(v, c) for (v, c) in zip(values, self.channels)]
 
+    def _load_channels(self):
+        self._fh.seek(0)
+        reader = csv.reader(self._fh)
+        header = reader.next()
+        self._fh.seek(0)
+
+        self.channels = []
+        for entry in header:
+            for mt in MEASUREMENT_TYPES:
+                suffix = '_{}'.format(mt)
+                if entry.endswith(suffix):
+                    site =  entry[:-len(suffix)]
+                    measure = mt
+                    name = '{}_{}'.format(site, measure)
+                    break
+            else:
+                site = entry
+                measure = 'unknown'
+                name = entry
+
+            chan = InstrumentChannel(name, site, measure)
+            self.channels.append(chan)
+
 
 class InstrumentChannel(object):