Callback output, split SDK

• Add a callback output for pull-based components without time step
• Add property is_push_based to IOutput to indicate if data must be pushed
• Handle pull-based inputs in ConnectHelper/try_connect
• Log warning when a time component was not updated during a run
• Split module sdk, as it exceeded 1000 lines of code
• Add tests

Fixes #22 (closed)

Pull-based components/outputs offer one more way to build non-working compositions. We should check for them! Continued in #49 (closed).

CHANGELOG.md

@@ -5,10 +5,14 @@
 ### Data and metadata rework
 
 * Add conversion between CRS to regridding adapters, using `pyproj` (!95)
-* Added more data tool functions: `quantify(xdata)` and `check_axes_uniformity(axes)` (!96)
+* Add more data tool functions: `quantify(xdata)` and `check_axes_uniformity(axes)` (!96)
 * In outputs, the name of the data is overwritten instead of failing the check (!98)
 * Adapters can pass through data with time, even if it does not match the pull time (which is quite common) (!98)
 
+### Interface
+
+* Add `CallbackOutput` for implementing pull-based components 
+
 ### Other  
 
 * Components are allowed to be in state VALIDATED at the end of a run (i.e. not updated) (!97)