Improve Association Matrix

From @verdoes inline on DRLD:

• Give external data items right color (already in #25 (closed)).
• Rename intermediate data into Recipe Data Products.
• Remove this from legend: "Data product repeated for clarity".
• Rename "RecipeStep" into "DRL Function" and
• give each DRL Function a number ID to avoid confusion about DRL Functions which are on the same height are sometimes the same and sometimes not.
• Give more detailed description in caption about the difference between the data flow from left to right (horizontal black lines) and the data flow from top to bottom (vertical black lines).