better docstrings (#144)

* first example of returns statement in docstring

* added a some return statements for pandas API

* last return statements in pandas input

* added returns to bdio docstrings

* few returns statements added to docstring

* finished docstrings for hadrons submodule

* also finished docstrings for json submodule

* finished misc submodule

* added returns in docstrings in openqQCD

* made some cosmetic chanes to dostrings

* added return nad return statement in docstring

* linting

* Improved docstrings of mpm, fits, roots, misc to have return statements

returns added for misc.py

returns added for mpm.py

reutrns added for fits.py

* linting...

* Some polishing of docstrings

pyerrors/mpm.py

@@ -21,6 +21,11 @@ def matrix_pencil_method(corrs, k=1, p=None, **kwargs):
         matrix pencil parameter which filters noise. The optimal value is expected between
         len(data)/3 and 2*len(data)/3. The computation is more expensive the closer p is
         to len(data)/2 but could possibly suppress more noise (default len(data)//2).
+
+    Returns
+    -------
+    energy_levels : list[Obs]
+        Extracted energy levels
     """
     if isinstance(corrs[0], Obs):
         data = [corrs]