Implement a layer syntax

   Layer system:
   [ <float>(nm|um|mm) Formula[<@Density>] [//  <float>(nm|um|mm) Formula<@Density>] // SubstrateFormaula[@<density>]#x*y*z ]

   layers can get grouped an repeated.
   x,y are substrate dimensions (mm)
   z is the substrate thickness (mm).

   Example:
     [( 1mm Fe[59] // 1mm Fe[58] )10 //  Fe[57]#5*5*20 ]
     10 times  (1mm Fe[59]/1mm Fe[58]) on  20 mm Fe[57] with a 5x5 mm sq area.