Release 0.3-beta JOSS Release Candidate

What's Changed

• Removed the use of git submodules. by @mparno in #177
• Removing submodules references by @dannys4 in #183
• 147 render notebooks into sphinx documentation by @bonh in #179
• Add Sphinx Tabs by @dannys4 in #187
• added stochastic volatility example to doc by @bonh in #189
• Fix tab problem by @dannys4 in #190

Full Changelog: v0.2.1-beta...v0.3-beta