Introduce exponnorm distribution

[ira-halenok]

As a business analyst, I want to be able to use exponnorm type of distribution.

Recently, we generalised usage of function distribution. As a result, pix-framework defines list of function distribution that could be used by an end-user. We have use cases when exponnorm is being used.

As of now, this type of distribution is not being supported by pix-framework. To fix this, we need to introduce changes to pix-framework itself. distribution.py should be changed to support a new type. In general, we need:

1. define what should be user-friendly parameters to this function
2. find formula to move from user-friendly parameters to the one, accepted by SciPy.

Additionally, when Prosimos started using generalised function distributions, some tests were commented out because of no support for exponnorm distribution. Once this feature is implemented, those tests should be uncommented. All seven tests that were commented could be found here.

As having commented tests is not the best code practice, I might suggest to put priority to this item (if that's possible).

[ira-halenok]

@orlenyslp @SimpleSquirrelz @david-chapela I've put on some details about the current situation of function distributions in pix-framework. We lack one of the function distribution exponnorm which is used for example in LoanApp_sequential_9-5.bpmn model. I don't know who should be responsible for implementing this, so I put all of you and you can decide later on. Important point here is that we need to uncomment ignored tests when exponnorm starts being supported.