repo_name must not contain hyphens or other illegal characters and project_short_name is missleading

Since repo_name is used as the last part of the generated java package name, it must adhere to the naming convention of package names. It also must be a valid part of a URL

project_short_name is misleading because its usage is unrelated to the project name.

Idea:

• Add a check in pre_gen_project.py for repo_name
• project_short_name should be renamed to grammar_name (and possibly be initialized with a valid value created from project_name by removing whitespace and other illegal characters)