Explicit exports

Created an explicit export list for the main DK module. This helps by hiding some helper functions.

It raises the question of whether to export the `Dimensional` constructor of `Dimensional`. I would strongly favor not exporting it to prevent shenanigans, but the current implementation has the benefit of allowing the `meter` etc. to be defined in the `SIUnits` module instead of in the main module.

It is possible to get the best of both worlds by making an `Internal` submodule.