Ewald summation in metallic slit

[mlund]

This adds ewald summation for slit geometries with metallic walls in the z-direction. Not yet ready for merging.

Progress:

• [x] New Ewald scheme with mirror charges incl. real<->mirror charge interactions.
• [x] Scaffold for new ewald policy incl. unittest (src/energy.h, src/energy.cpp)
• [x] Add METALSLIT option for docs/schema.yml
• [x] Fill in scaffold for policy
• [ ] Include mirror charges in surface energy
• [ ] Add unittests for policy
• [ ] Complete examples/metal-slit-ewald
• [x] Optimize energy::MetalSlitEwald::energy() for partial system updates
• [ ] Update manual, docs/_docs/energy.md