Update citation info to SciPost publication

[goerz]

As soon as https://arxiv.org/abs/1902.11284 is published in SciPost, the instructions on citing krotov should be updated. Specifically:

• [x] Update __citation__ variable in src/krotov/__init__.py
• [x] Update __bibtex__ variable in src/krotov/__init__.py
• [x] Update section "Citing the Krotov Package" in docs/01_overview.rst
• [x] Replace the arxiv badge in README.rst and docs/index.rst with a DOI badge that links to https://doi.org/<DOI> (pointing to the SciPost article)
• [x] Update the READMEs in the paper branch