added some function writing documentation

sphinxdoc/documentation/WritingFunctions.rst

+Writing Functions
+=================
+
+Writing non-standard functions
+------------------------------
+
+When implementing non-standard functions, i.e. all function not decorated with ``flagging``, some special care is
+needed to comply to the standard ``SaQC`` behaviour. The following passages guide you through the jungle of
+``register`` arguments and there semantics.
+
+Masking
+"""""""
+TODO
+
+Demasking
+"""""""""
+TODO
+
+History squeezing
+"""""""""""""""""
+TODO
+
+Target handling
+"""""""""""""""
+Functions decorated with ``register(handles_target=True)`` are fully in charge to implement the source-target
+workflow. That means they need to handle the following cases:
+
+- Existing ``target``: one ore more of the given targets might already exist in ``data`` and ``flags``.
+  There is (currently) no hard rule how we expect functions to handle this case, the default functions
+  however drop pre-existing ``target`` with:
+
+  .. code-block:: python
+
+     from saqc.funcs import dropField
+
+     data, flags = dropField(data=data, fags=flags, field=field, target=target)
+
+- Non-existing ``target``: we stick to the idea, that non existing targets will be handled
+  gracefully. Usually that implies, that after a function returns all names given in ``target`` exist
+  in ``data`` as well as in ``flags``. The easiest solution here is to create an explicit copy:
+
+  .. code-block:: python
+
+     from saqc.funcs import copyField
+
+     data, flags = copyField(data=data, flags=flags, field=field, target=target)
+
+  However, such a direct mapping/lineage between ``field`` and ``target`` is not always meaningful. Usually
+  the function itself either provides new ``flags`` or new ``data``, but we need to also provide values for
+  for the respective other structure. For ``data`` we default to a series with all ``np.nan`` values, e.g.
+
+  .. code-block:: python
+
+     data[target] = pd.Series(np.nan, index=data[field].index)
+     
+  and empty ``flags``, e.g.
+
+  .. code-block:: python
+
+     from saqc.core import History
+     flags.history[target] = saqc.core.history.History(data[target].index)
+  
+- Incompatible lengths of ``field`` and ``target``: Certain functions might have specific mapping
+  needs, some multivariate functions, for example, might map exactly three ``fields`` to one ``target``,
+  or need exactly on ``target`` for every field. If this is the case, such invriants should be checked
+  accordingly.
+
+- Incompatible indices for ``field`` and ``target``: Depending on the function, differing indices for
+  ``field`` and ``target`` might or might not impose problems for the implementation. If not coded carefully,
+  chances are however high, that this might cause breakage. It is therefore recommended to check this
+  accordingly.
+  

sphinxdoc/index.rst

@@ -81,6 +81,7 @@ Getting Started
    :maxdepth: 1
 
    Documentation Guide <how_to_doc/HowToDoc>
+   Writing Functions <documentation/WritingFunctions>
 
 
 Indices and tables