Fix examples
Merge pull request #7508 from smichr/atoms

use free_symbols instead of atoms(Symbol) where appropriate
Add Zamrath Nizam to AUTHORS/aboutus. Welcome to SymPy!
Add Maciej Baranski to AUTHORS/aboutus. Wecome to SymPy!

also added .mailmap entry
added sampling_density to sympy.stats
Add Benjamin Gudehus to AUTHORS/aboutus. Welcome to SymPy!
make sampling_density tests more robust
7456: make random_symbols canonically ordered
4600: clarify documentation in test_dimacs.py
add free_symbols method to MatrixBase

This allows Matrix(...).free_symbols to work now.
Merge pull request #7515 from smichr/density-numsamples

Added sampling_density to stats
add sampling_density to stats import
Merge pull request #7516 from smichr/sample

add sampling_density to stats import
Merge pull request #7506 from debugger22/landscape-io

An attempt to minimize errors on landscape.io
add free_symbol test for all matrix types
Merge pull request #7509 from smichr/free

add free_symbols method to MatrixBase
4873: fix link to geometry plotting
Enables ccode(..., user_functions={..., 'Pow': ...})
Merge pull request #7511 from smichr/doc

4873: fix link to geometry plotting
Merge pull request #7517 from bjodah/allow_user_pow_ccode

Enables ccode(..., user_functions={..., 'Pow': ...})