Feature/specific treatment for qgis dataset names

closes #99 (closed)

Split QgsMeshLayer dataset name in 2 when dataset is vector:

• special treatment for vitesse and velocity variables. We split dataset with u/v
• for other use a split variable : along / suivant / u,v

Add options to :

• upper all variable names (MDAL lower all variable but selafin can store value in lower and upper)
• define option for vector dataset split : along / suivant / u,v