DDC-1004: Option for mapping:import to not write Entity Files if they exist

[doctrinebot]

Jira issue originally created by user monofone:

There should be an option to omit the creation of allready existing entities:

Use Case:

if you develop your model in the database and create the entities with the cli and then go on with development and create new tables you would build the entities again with the cli. This will kill your allready existing and possible extended entities.

Best would be if you have to set the option when you want the existing entities overwritten. so you cant accidently overwrite them

[billschaller]

There are a number of options for the entity generator that now cover this.