Improve test coverage and add object mockers

[jacopoabramo]

As correctly pointed out by @kasasxav in #166, we need to improve code coverage and add object mockers.

What I'm thinking for mockers though is that your testing with the Hamamatsu mock forces developers to integrate a mock object for each device manager that they implement (i.e. here) . It doesn't seem ideal to me. I would instead consider the possibility to load the mockers withing each specific DevicesManager (DetectorsManager, PositionersManager, etc.) so that when an exception is thrown the mock is loaded for that device.

It moves the implementation to the higher stack and relieves the stress of developers to implement their own everytime.