Support vectors in differential equations

[pnbabu]

Currently, vectors are not supported in the equations block in a NESTML model. The equations should be able to use and index the vector variables declared in the state or parameters block.

For example:

state:
  k_step integer = 0
  ..
end

equations:
  ...
  V_m' = ( -I_leak - I_syn_exc - I_syn_inh + I_e + I_stim + I_step[k_step] ) / C_m
end

parameters:
  ...
  n_step integer = 1
  I_step[n_step] pA = 0. pA
end