organizing the docs - put the content in a content folder. put the SimPEG core api docs in core_api

docs/content/api_core/api_DataMisfit.rst

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+.. _api_DataMisfit:
+
+
+Data Misfit
+***********
+
+The data misfit using an l_2 norm is:
+
+.. math::
+
+    \mu_\text{data} = {1\over 2}\left| \mathbf{W}_d (\mathbf{d}_\text{pred} - \mathbf{d}_\text{obs}) \right|_2^2
+
+If the field, u, is provided, the calculation of the data is fast:
+
+.. math::
+
+    \mathbf{d}_\text{pred} = \mathbf{Pu(m)}
+
+    \mathbf{R} = \mathbf{W}_d (\mathbf{d}_\text{pred} - \mathbf{d}_\text{obs})
+
+Where P is a projection matrix that brings the field on the full domain to the data measurement locations;
+u is the field of interest; d_obs is the observed data; and \\\(\\mathbf{W}_d\\\) is the weighting matrix.
+
+The derivative of this, with respect to the model, is:
+
+.. math::
+
+    \frac{\partial \mu_\text{data}}{\partial \mathbf{m}} = \mathbf{J}^\top \mathbf{W}_d \mathbf{R}
+
+The second derivative is:
+
+.. math::
+
+    \frac{\partial^2 \mu_\text{data}}{\partial^2 \mathbf{m}} = \mathbf{J}^\top \mathbf{W}_d \mathbf{W}_d \mathbf{J}
+
+
+The API
+=======
+
+.. autoclass:: SimPEG.DataMisfit.BaseDataMisfit
+    :members:
+    :undoc-members:
+
+Common Data Misfits
+===================
+
+l2 norm
+-------
+
+.. autoclass:: SimPEG.DataMisfit.l2_DataMisfit
+    :members:
+    :undoc-members: