'Adds a display app to the datatype.
app_id is a unique id
label is the primary display label, e.g., display at \'UCSC\'
file_function is a string containing the name of the function that returns a properly formatted display
links_function is a string containing the name of the function that returns a list of (link_name,link)'

def add_display_app(self, app_id, label, file_function, links_function):

    self.supported_display_apps = self.supported_display_apps.copy()
    self.supported_display_apps[app_id] = {'label': label, 'file_function': file_function, 'links_function': links_function}
