Create a measurements service

[dannyrb]

RE: https://github.com/OHIF/Viewers/pull/954/files#r354547871

Measurements Service

Context

Per a recent change, we now have the ability to persist supported annotations with DICOM Structured Reports. We also have the ability to parse and display DICOM Segmentations. When our application parses this information, it stores it in cornerstone-tools tool state.

This has the unwanted side-effect of causing every consumer of annotation/seg data to depend on cornerstone-tools, and to use it's API when it needs to read/modify measurements.

We would also like to expand the functionality of measurements, but modifying cornerstone-tools to do so is not always appropriate.

Guidance

If we create a MeasurementService, it will be made available to our application and extensions via the ExtensionManager and ServicesManager. A potential implementation of this would allow:

• An internal representation of measurements
  • See Slicer, cornerstone-tools, and vtk.js widgets for examples
• A way to add/update/delete measurements
  • Path for cornerstone-tools and DICOM SR to push
• A way to query for one or all measurements
  • Get data out for persistence
• A way to subscribe to updates
  • Observables w/ proxies? Simple pub/sub? Callbacks?

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We've recently created our first set of Services. You can read more about them here:

https://docs.ohif.org/services/

[dannyrb]

Some additional requirements spelled out here: #1314