[RF] New PDF: gaussian with double sided exponential tails

[ferdymercury]

This Pull request:

Changes or fixes:

Fixes https://its.cern.ch/jira/browse/ROOT-8093

It's a simpler alternative to RooCrystalBall with potentially less instabilities

Checklist:

• tested changes locally
• updated the docs (if necessary)

[github-actions]

Test Results

    20 files      20 suites   4d 18h 45m 43s ⏱️
 2 726 tests  2 726 ✅ 0 💤 0 ❌
52 636 runs  52 636 ✅ 0 💤 0 ❌

Results for commit 691e35c.

♻️ This comment has been updated with latest results.

[mpuccio]

Hello! Thanks a lot for this, it is extremely useful. About the naming: this distribution has been used in ALICE since well before the paper from Souvik Das, see for instance https://www.worldscientific.com/doi/epdf/10.1142/S0217751X14300440 figure 42 and some code implementation here that was published in 2015. Just to say that it might be fine to just call it for what it is and avoid name attributions.