Update dependency list

Closes #247
Closes #246
Merge pull request #257 from ChrisBeaumont/histo-log-tick

Histogram improvements
Merge pull request #256 from ChrisBeaumont/deps

Update dependency list
Yield-able state loaders to handle circular imports
Refactor SubsetState to remove parent reference
Refator DataCollection to instantiate the hub everyone is to use
Move Pointer class
Implement Subset Groups
Subset Group Style overriding logic
Pull up property
New M/V implementation of Data Collection View
Make DataCollectionModel read-only, but auto-synced with underlying Data
Integrate DataCollectionModel into gui. UNFINISHED
Update LayerTreeWidget and EditSubsetMode for SubsetGroups.
UI tweaks to handle SubsetGroups gracefully
Tweak how style changes behave and are presented in UI
Remove old data_collection_view
DataCollectionModel selection and flag fixes
facet_subsets uses subset groups
SubsetGroup state saving/loading