Feature/conda based travis ci

[boltbeard]

The essential idea of this feature was to allow the use of conda within travis' CI. The .travis.yml file has been change appropirately but somehow during the installation of conda and the solving of the environment, it install Python 2.7 and not the expected Python version.

The current state of this is that: it is unknown why Python 2.7 is defaulted to eventhough there is an explicity python verision given at the beginning.

Hypothesis: There is a chance that if the version of python is not mentioned in the conda yml file which it uses to creat the environment, "conda doesn't take it seriously enough!" and defaults to using Python 2.7

[gplssm]

Now, Travis CI uses conda for installing ding0 dependencies and running the tests.

Furthermore, the branch fix/test_grids is shipped by this PR introducing more tests.

Since ding0 is equipped with 56 tests, this PR also closes #86.

Before the merge can be done, following tasks remain

• review newly introduced tests from fix/test_grids
• Eventually update "What's new"
• Check if docs build
• Check if docs need an Update

[nesnoj]

I think this one is outdated, it might be helpful if we switch to github actions #338 but I'm not sure if it's worth to re-integrate after 2 years.. What do you think @gplssm?