Add docs stubs for threshold tracking

docs/source/identification.rst

@@ -8,6 +8,7 @@ Some of the identification techniques we provide include:
 
    identification_african_easterly_waves
    identification_pv_streamer
+   identification_threshold
    identification_overlap_example
    identification_template
 

docs/source/identification_threshold.rst

+Threshold-based
+===============
+
+...
+
+.. autoclass:: enstools.feature.identification.threshold.SingleThresholdIdentification
+.. autoclass:: enstools.feature.identification.threshold.DoubleThresholdIdentification
+

docs/source/tracking.rst

@@ -8,6 +8,7 @@ Some of the tracking techniques we provide include:
 
    tracking_african_easterly_waves
    tracking_overlap
+   tracking_overlap_double_threshold
    tracking_template_object_compare
    tracking_template
 

docs/source/tracking_overlap_double_threshold.rst

+Overlap with Double Threshold
+=============================
+
+...
+
+.. autoclass:: enstools.feature.tracking.overlap_double_threshold_tracking.OverlapDoubleThresholdTracking
+