v0.3.0

PR's since last release:

• Limit pmd params, and advance two separate modes #84 by @sajjadazimi
• Ej/add_superparticles #85 by @edejong-caltech
• Updating examples #87 by @sajjadazimi
• Ej/equation_types #96 by @edejong-caltech
• Bors is dead #99 by @edejong-caltech
• Try updating Optim to 1.6 and newer #112 by @edejong-caltech