Fix errors in model class     -   Add networks dict to record all available nets and their display names     -   Do not compute activations for None layers and the like     -   Fix the `setUnit()` method to set object property instead of returning     -   Fix a few porperty references     -   call `setInputData()` when a new source is selected to compute activations         at index 0     -   Enforce list input `in activations_for_layers()`     -   Add `get_input()` utility to obtain an input image at a given index
Fix errors in model class
    -   Add networks dict to record all available nets and their display names
    -   Do not compute activations for None layers and the like
    -   Fix the `setUnit()` method to set object property instead of returning
    -   Fix a few porperty references
    -   call `setInputData()` when a new source is selected to compute activations
        at index 0
    -   Enforce list input `in activations_for_layers()`
    -   Add `get_input()` utility to obtain an input image at a given index

Petr-By/qtpyvis