[WIP] GUI synaptic gains implementation

[gtdang]

Added synaptic gains widgets to the GUI.

Changes:

1. Adjusted Network class methods 2. Renamed update_weights method to set_synaptic_gains for more clarity 3. Added get_synaptic_gains method and test
2. Added new "Synaptic gains" child tab under the Network tab with widgets for each weight
3. Updated the _init_network_from_widgets function to update synaptic gains
4. Added tests for GUI synaptic gains
5. Corrected spelling error for cell parameters variable