Make IqtTemplateFunctionModel and ConvTemplateFunctionModel similar and share code

[robertapplin]

Description:
The IqtTemplateFunctionModel and ConvTemplateFunctionModel should be refactored to make them more generic and to allow them to share code within the MultiTemplateFunctionModel class. This should make it easier to add fit functions to the tabs which use this class in the future. There are a few things I don't like in the current code:

• ConvTemplateFunctionModel uses a very specialised method to construct its MultiDomainFunction
• Both tabs have prefixes hard-coded into the code. We should be able to retrieve them from the MultiDomainFunction stored in the model
• ConvTemplateFunctionModel uses a slightly different mechanism to set Q values in Q-dependent functions

Acceptance Criteria

• The IqtTemplateFunctionModel and ConvTemplateFunctionModel use much the same code
• Prefixes are retrieved from the MultiDomainFunction in the model
• Both use the same method for constructing their fit function
• The TemplateSubTypes are no longer hard coded within these models