Legacy dataset service

[bnorthan]

LegacyDatasetService overrides getDatasets() such that all ImageDisplays will be lazily registered when the user calls getDatasets(). All other methods simply wrap a call to the next highest priority DatasetService.

LegacyDatasetService follows the pattern used for LegacyImageDisplayService.

@ctrueden, @hinerm the code that registers the datasets is in the function registerDatasets(). Let me know if that look OK.

My understanding is that this service will be made obsolete by #105. If that is not the case let me know. If this code has a long-term shelf life than there is a couple more things I should do (like putting similar code for LegacyDatasetService and LegacyImageDisplayService in a shared utility).

[ctrueden]

Thanks for following up with this, @bnorthan. There are some tweaks I'd like to do with the patches; I'll try to work on it next week some time.

[imagejan]

@ctrueden is this PR still relevant?

[ctrueden]

Thanks for the ping. I cannot remember. I will bookmark this to resolve some time soon, by some definition of "soon."

[ctrueden]

By the definition of "soon" where it is one year and two days later: I now close this, since #182 was merged. If anyone has reason to think these changes are still necessary, please reopen and/or file another PR accordingly.