Bugfixes, some features

Added openMP in configure on supported platforms (including macOS using homebrew).
Added working slice modifier.
Readding fix atomify after clear command.
Updated LAMMPS to 24Jul17.
Improved Rendering tab.
Removed .jpg as screenshot alternative (now defaulting to .png).
Added close button on RightBar text about Atomify.
Fixed shortcut text multiple places.
Added reax examples.
Updated and improved other examples.
Translate guides so positive direction can be identified.
Added lost_atoms detection in fix_atomify.
Not showing empty message overlay when stopped is clicked.
Attenuation calculation follows global scale.