Refactor of the metocean API.

This PR is the first refactor of the metocean API.

It contains the first draft of minimal changes required to abstract the internal mapping representations for the user.

It does not consider value maps, which will be address in another additive PR, nor does it contain unit tests, which will be added once a test framework is established with another PR.

This PR introduces the dependency on ```pydot``` for digraph plotting of MetaRelate mappings.