First implementation of the French INRETS Volume Delay function.
Beta is not used but kept for better consistancy across VD functions.

First implementation of BPR2 Volume Delay function.
This doubles beta above capacity so that fewer vehicles are affected when capacity is exceeded.
Double Beta insteed of using a Beta' allow to use only 2 parameters as for other VD functions. Integration into QGIS GUI is also easier 😜

Correction of style issues base on flake8 for .py files (E501, line too long , is ignore).
Do we need to correct all E501 ?
Anything to check for pyx and rst files ?