added VolumeTreeNode to Stack

See merge request !78

Resolve "Require python 3 in cmake"

Closes #123

See merge request !77

Resolve "Flexible Stack"

Closes #19

See merge request !73

Resolve "add constant for nucleon mass"

Closes #122

See merge request AirShowerPhysics/corsika!75

Resolve "add PDG to CORSIKA id conversion"

Closes #125

See merge request !74

Resolve "use nuclear stack extension in nuclear interaction"

Closes #133

See merge request AirShowerPhysics/corsika!72

Resolve "Need "StackView" object"

Closes #106

See merge request AirShowerPhysics/corsika!71

Resolve "add Nucleus to corsika id conversion"

Closes #126

See merge request AirShowerPhysics/corsika!70