Delta-Delta Tests and Docs

1.  added doc string to class DeltaDelta and corrected typos
2. changed delta-delta tests and utils.py to mutant-drug configuration
3. generated test images for delta-delta tests
4. added tutorial page deltadelta.rst to include calculations and example plots
5. modified index.rst to include deltadelta
6. modified api.rst to include deltadelta
7. updated tutorial images for deltadelta

Merge pull request ACCLAB#105 from josesho/v0.3.1

v0.3.1

Merge pull request ACCLAB#96 from ACCLAB/v0.3.0

v0.3.0

Merge branch 'v0.2.8' of https://github.com/ACCLAB/DABEST-python into…

… v0.2.8

update docs

Merge pull request ACCLAB#77 from ACCLAB/v0.2.6-dev

v0.2.6

Merge pull request ACCLAB#65 from ACCLAB/v0.2.5

v0.2.5

Update release-notes.rst

Fix typo

Merge pull request ACCLAB#36 from josesho/v0.2.3

v0.2.3

Merge pull request ACCLAB#34 from ACCLAB/v0.2.2

v0.2.2