v0.9.0 -- GR

Notes

1. Full GR field solver/current deposit/particle pusher
2. gui implementation is more refined
3. optional external force driver enabled with -D extforce=ON
4. minor bugs fixed + restructuring of the code
5. updated dependencies

What's Changed

• Dev/output by @haykh in #19
• GR Support v0.9 by @haykh in #17
• Dev/bc by @haykh in #20
• Dev/nttiny by @haykh in #24
• GR v0.9 by @haykh @bcrinquand @alisagk in #22
• Dev/extforce by @munizhou @jmahlmann in #25

Full Changelog: v0.8.5...v0.9