Introduce String types in EMA

• make it possible to use String as a type for ports and component parameters
• write tests
• This reference model of a machine learning pipeline would parse when trying to provide String parameters in component definitions, even though no explicit support for Strings seems to take place. However, trying to instantiate the corresponding component, here instance Data_Access ("path.to.datasource") data_access_step; would lead to a parsing error. To reproduce this, it is sufficient to try to parse the model with the parsing API provided by the EMA language.