Add an evaluate_mappings function to PulseTemplate

[shumpohl]

A user that wants to set a parameter to a specific value or map a channel name uses a MappingPT either directly or with the with-mapping method. This is very useful for composite pulse templates where the named components are serialized in individially and can be independently changed.

However, this introduces an overhead for runtime generated and evaluated pulse templates where this functionality is not needed. This issue proposes a method evaluate_mappings which returns a new pulse template without MappingPTs which do not introduce new parameters.

def evaluate_mappings(self, parameters=None, channel_mapping=None, measurement_mapping=None):
    ...