Documentation/DerivedMeasurements: Added documentation for new API
diff --git a/doc/derived_measurements.rst b/doc/derived_measurements.rst
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+Derived Measurements
+=====================
+
+
+The ``DerivedMeasurements`` API provides a consistent way of performing post
+processing on a provided :class:`MeasurementCsv` file.
+
+Example
+-------
+
+The following example shows how to use an implementation of a
+:class:`DerivedMeasurement` to obtain a list of calculated ``Measurements``.
+
+.. code-block:: ipython
+
+ # Import the relevant derived measurement module
+ # in this example the derived energy module is used.
+ In [1]: from devlib import DerivedEnergyMeasurements
+
+ # Obtain a MeasurementCsv file from an instrument or create from
+ # existing .csv file. In this example an existing csv file is used which was
+ # created with a sampling rate of 100Hz
+ In [2]: from devlib import MeasurementsCsv
+ In [3]: measurement_csv = MeasurementsCsv('/example/measurements.csv', sample_rate_hz=100)
+
+ # Process the file and obtain a list of the derived measurements
+ In [4]: derived_measurements = DerivedEnergyMeasurements.process(measurement_csv)
+
+ In [5]: derived_measurements
+ Out[5]: [device_energy: 239.1854075 joules, device_power: 5.5494089227 watts]
+
+API
+---
+
+Derived Measurements
+~~~~~~~~~~~~~~~~~~~~
+
+.. class:: DerivedMeasurements()
+
+ The ``DerivedMeasurements`` class is an abstract base for implementing
+ additional classes to calculate various metrics.
+
+.. method:: DerivedMeasurements.process(measurement_csv)
+
+ Returns a list of :class:`Measurement` objects that have been calculated.
+
+
+
+Available Derived Measurements
+-------------------------------
+.. class:: DerivedEnergyMeasurements()
+
+ The ``DerivedEnergyMeasurements`` class is used to calculate average power and
+ cumulative energy for each site if the required data is present.
+
+ The calculation of cumulative energy can occur in 3 ways. If a
+ ``site`` contains ``energy`` results, the first and last measurements are extracted
+ and the delta calculated. If not, a ``timestamp`` channel will be used to calculate
+ the energy from the power channel, failing back to using the sample rate attribute
+ of the :class:`MeasurementCsv` file if timestamps are not available. If neither
+ timestamps or a sample rate are available then an error will be raised.
+
+
+.. method:: DerivedEnergyMeasurements.process(measurement_csv)
+
+ Returns a list of :class:`Measurement` objects that have been calculated for
+ the average power and cumulative energy for each site.
+
+
diff --git a/doc/index.rst b/doc/index.rst
index 2c6d72f..5f4dda5 100644
--- a/doc/index.rst
+++ b/doc/index.rst
@@ -19,6 +19,7 @@
target
modules
instrumentation
+ derived_measurements
platform
connection