Use enums instead of custom dictionaries where possible

[EZoni]

I think the code would be more readable and robust if we used enums instead of custom dictionaries where possible.

One use case that I think would be ready to switch is the ML model type. For example, this

class ModelType(Enum):
    neural_network_single = "Neural Network (single)"
    neural_network_ensemble = "Neural Network (ensemble)"
    gaussian_process = "Gaussian Process"

would allow to stop relying on string checks like if model_type == "NN", where the correspondence between "NN" and "Neural Network (single)" is defined in a custom dictionary, currently

synapse/dashboard/model_manager.py

Lines 19 to 23 in 6d135b2

	model_type_tag_dict = {
	"Gaussian Process": "GP",
	"Neural Network (single)": "NN",
	"Neural Network (ensemble)": "ensemble_NN",
	}

and replace them with if model_type == ModelType.neural_network, while still leaving access to a user-friendly string representation to be exposed in the UI (e.g., "Neural Network (single)").

Something similar probably can be done for other variables related to selectors and other UI components.