Parse PKMODEL BLOCK data
PKMODEL(
  ncmt = 1,
  depot = FALSE,
  cmt = NULL,
  trans = pick_trans(ncmt, depot),
  env = list(),
  pos = 1,
  ...
)number of compartments; must be 1 (one-compartment, not including a depot dosing compartment) or 2 (two-compartment model, not including a depot dosing compartment)
logical indicating whether to add depot compartment
compartment names as comma-delimited character
the parameterization for the PK model; must be 1, 2, 4, or 11
parse environment
block position number
not used
When using $PKMODEL, certain symbols must be defined in the 
model specification depending on the value of ncmt, depot 
and trans.
ncmt 1, depot FALSE, trans 2: CL, V
ncmt 1, depot TRUE , trans 2: CL, V,  
KA
ncmt 2, depot FALSE, trans 4: CL, V1, 
Q, V2
ncmt 2, depot TRUE , trans 4: CL, V2, 
Q, V3, KA
If trans=11 is specified, use the symbols listed above for the 
ncmt / depot combination, but append i at the end 
(e.g. CLi or Qi or KAi).
If trans=1, the user must utilize the following symbols:
pred_CL for clearance
pred_V  or pred_V2 for central compartment volume of 
distribution
pred_Q  for intercompartmental clearance
pred_V3 for for peripheral compartment volume of distribution
pred_KA for absorption rate constant