bump: version 0.1.8.post1 -> 0.1.9

Merge pull request #16 from openscm/FORD_play

Added documentation of the Fortran back-end/API using [ford](https://forddocs.readthedocs.io/en/stable/).
Integrated the generated FORD docs into MkDocs so that the overall documentation is unified, searchable, and easy to navigate. The combined result looks solid.

bump: version 0.1.7.post1 -> 0.1.8

bump: version 0.1.6.post1 -> 0.1.7

bump: version 0.1.5.post1 -> 0.1.6

bump: version 0.1.4.post1 -> 0.1.5

bump: version 0.1.3.post1 -> 0.1.4

bump: version 0.1.2.post1 -> 0.1.3

bump: version 0.1.1.post1 -> 0.1.2

bump: version -> 0.1.1