Revise speaker attributes and add XML example

[klinzey]

Updated speaker attributes table and added examples for clarity.

[petrvanekrobe]

As per our discussion:

• Speaker geometry might be empty
• The data of the speaker should be for the whole speaker box, represented by the speaker geometry, which is defining the audio emittion point. Speaker emits its sound into negative Y-direction (Z-up).
• Combine MaxSPL and it's attributes .