Simplify code for sx0_fixedParameters (#1300)
Simplify code for sx0_fixedParameters (#1300)

* Simplify code for sx0_fixedParameters

Simplify code for `sx0_fixedParameters` by only including non-zero entries in switch statement.

For performance test model, this reduces file size from ~32MB to < 200KB and speeds up compilation:

* gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
  
  `-O2`

  before: User time (seconds): **112.97**, Maximum resident set size (kbytes): **2502824**
  now: User time (seconds): **0.37**, Maximum resident set size (kbytes): **75636**

* icc (ICC) 19.0.5.281 20190815

  `-O3`

  before: killed after **5h**
  now: User time (seconds): **1.87**, Maximum resident set size (kbytes): 228984

Closes #1297

AMICI-developer/AMICI,FFroehlich/AMICI,FFroehlich/AMICI,FFroehlich/AMICI,FFroehlich/AMICI,AMICI-developer/AMICI,AMICI-developer/AMICI,AMICI-developer/AMICI