Refactor `load_all_datasets` to allow for a different location for `datasets`

[xrotwang]

It may be only me, but I think it is a bit weird that we clone dplace-data into a subdirectory of dplace. For all my other projects I have app-code-repos and data repos side by side. Is anyone against refactoring load_all_datasets accordingly? I'd also like to rip out the automatic cloning/updating of the data repos from this script. I think this is way too implicit. E.g. for the productive app, I'd rather have the load work with a exported release of the data repos and not with HEAD of the master branch.

[SimonGreenhill]

+1

[Bibiko]

+1