Specificiation of MP_EXP length/type

[markusa]

    Details as length and type of data remain to be defined according to
    the foreseen use by the experimenters.

• Isn’t type specified here?
• But length is specified also as <=255 surely? or does var imply some other format, the format of length MUST be defined.

[markusa]

Suggestion:

Replace Details as length and type of data remain to be defined according to the foreseen use by the experimenters. with According to the foreseen use by the experimenters Data MUST be defined within the scope of the 8bit Length field.

See #219