Take non_linear as input argument

[EBHolm]

In CLASS v3.0, the input non linear was renamed to non_linear. Recent versions of cobaya set a default value for non_linear, which immediately throws an error when using TriggerCLASS v6.1 with the new cobaya versions.

One could fix this by changing the cobaya behaviour, but I think the simplest fix is to allow for either of non linear or non_linear as arguments in TriggerCLASS.