Broken message bit index calculation

[jklmnn]

Currently the bitindex of a field in a message is calculated inconsistently depending on the context. Most calculations calculate the index of a field by the predecessors first and length. They usually rely on Initial'First being set which is set to 0 in the constructor. This creates two problems:

• Message'First must be greater than 0
  This results in all calculations made with Initial'First being incompatible to Message'First
• Message'First is not handled
  Since Initial'First is used as base any change to Message'First by the model might be ignored, and if it is not the results of many calculations are probably wrong

To fix this Initial'First should be set to Message'First and a mechanism is needed to detect if Message'First has been set in the message and to use either this or the default value 1.

[jklmnn]

As discussed Message'First should not be set in the model and can be implementation defined.What is left to do it so set Message'First = Initial'First = 0.

[jklmnn]

Fixed with #501.