InitialAssignments not supported in code generator

[matthiaskoenig]

Currently the initial assignments do not work in the code generator.

[matthiaskoenig]

Handle this correctly in the markdown between InitialAssignments and InitialValues.
E.g.

## Initial conditions `x0`
ald = <libsbml.ASTNode; proxy of <Swig Object of type 'ASTNode *' at 0x7f745d4aba20> >  # [mmol/l] aldosterone in Vplasma  
ang1 = <libsbml.ASTNode; proxy of <Swig Object of type 'ASTNode *' at 0x7f745d4aabe0> >  # [mmol/l] angiotensin I in Vplasma  
ang2 = <libsbml.ASTNode; proxy of <Swig Object of type 'ASTNode *' at 0x7f745d4abb40> >  # [mmol/l] angiotensin II in Vplasma  
anggen = 1e-08  # [mmol/l] angiotensinogen in Vplasma  
e3174 = 0.0  # [mmol/l] e3174 in Vplasma  
ren = <libsbml.ASTNode; proxy of <Swig Object of type 'ASTNode *' at 0x7f745d4abba0> >  # [mmol/l] renin in Vplasma