Consistent naming of xml file types

[johkoenig]

Currently, the word "model" is used twice:

• The xml description of a version of the REXS standard is called a "model", e.g. rexs_model_1.6.xml
• A description of a gearbox, i.e. a REXS file, is called a "model", e.g. in the class RexsModel

This can lead to some misunderstanding as well as some overlapping class names and descriptions. I propose the following new wording, which then also needs to be rolled to the publich database:

• A single REXS file myGearbox.rexs is called a "REXS Model" or short "Model". Consequently, the name of the class RexsModel can be kept, as it represents a single model of an actual gearbox.
• The XML files defining a version of REXS will be called "REXS Schema". This means, that:
  • Files like rexs_model_1.6.xml should be renamed to rexs_schema_1.6.xml
  • Classes like DbModelRegistry need to be renamed to RexsSchemaRegistry
  • The packages and folders called db should be renamed to schema
  • The public database and help resources should be rewritten to consequently use the term "REXS Schema".
• For completeness: The correct term for all XSD files should be "XML Schema" or in short just "XSD". Folders and packages containing/handling XSDs should be renamed to xsd.

To-Dos (@karstenroethig):

• Adjusting the REXS Database to export files with as rexs_schema_X.X.xml
• Adjusting the wording in the REXS Database to state "Rexs Schema" everywhere
• Adjusting the wording in the help files and documentation to state "Rexs Schema" everywhere
• Renaming the exported files from the REXS Database (in this repository)
• Renaming classes DbModelRegistry (and connected) to RexsSchemaRegistry
• Adjusting the package names from db to schema (on various locations)
• further tbd