Always call strategy.update() when a strategy attaches to a sensor.

This means that any sensor class that sets their OBSERVE_UPDATES attribute to
True will automatically send an initial update by calling the super class
start(). This is also compatible with the way that katcore.proxy_base replaces
dummy sensors.
Merge pull request #23 from ska-sa/minor-sampling-cleanup

Always call strategy.update() when a strategy attaches to a sensor.