feature: add possibility to specify collimator asymmetry factor

We add the possibility to specify an asymmetry factor for the collimators. The asymmetry factor multiplies the top half-gap to get that of the bottom layers. If it is zero, the collimator has a single jaw.

This parameter is used to remove the check on the TCDQ* collimators

We modify accordingly the json dataset in the example.

The new code has been tested and the results still agree well with the old model: