OneSdsc usability: specifying locations (popeye/SDSC) by kwarg, rest caching disabled by default

[grg2rsr]

The OneSdsc class does, as it is now, not work out of the box for datauser on SDSC because of differences in the default cache dir.

This patch:

• exposes a new keyword argument to select the location SDSC vs popeye, and handles the setting of the correct cache dir
• disables rest caching by default. Currently, cache_rest=None has to be passed when using OneSdsc on popeye (as users on popeye do not have write permission on the cache directory). Datauser on SDSC does, and rest caching can be enabled by setting a kwarg explicitly if it is required.