Optimize merlin interface (#70)

* Cache state of set merlin values

This avoids socket interface overhead of setting the same acquisition
time and number of frames over and over again

* Add dedicated methods for GET commands over socket

* Refactor set/get/cmd code to make it more robust

* Add skip argument to load first n bytes in frame

* Refactor test code

* Implement soft trigger function

* Optimize overhead for movie acquisition

* Debug exposure and set large number of frames to collect

* Set up seamless transition to soft trigger mode