wip: add double-sided Gaussian with exponential tails PDF and `CBExGauss` factory

[ikrommyd]

Early PR to add double-sided Gaussian with exponential tails PDF and CBExGauss factory

Proposed Changes

• add double-sided Gaussian with exponential tails PDF and factory to construct CBExGauss PDF from this and the GeneralizedCB. Implementation from: https://github.com/cms-egamma/egm_tnp_analysis/blob/master/libCpp/RooCBExGaussShape.cc

Tests added

• Modify tests/serialize/test_basics.py and tests/test_z_numpy.py accordingly and add tests/test_pdf_gaussexptail.py

Checklist

• [ ] change approved
• [x] implementation finished
• [x] docs updated
• [x] correct namespace imported
• [x] tests added
• [x] CHANGELOG updated
• [x] (if new public method/class) docs in rst file updated?
• [x] (if large change) tutorial added/updated?