* Add ISL type name in the Field to call read/write APIs for
symbol/string or blob/clob accordingly.
* Adds manifest directory in the path for the templates

* Adds `CodeGenerator::generate_code_for_authorities` which generates
code for each schema in given authorities
* Makes `--schema` optional for code generation

* Removed Import statements from template as those were used for
reference to other generated data models
* Removes mod.templ as the generated code now uses single module
* Removes generating `ion_data_model` module for generated code instead
uses a single file `ion_generated_code`