Add more samplers

[MichaelClerx]

Making an overview here, feel free to correct this list!

• [x] Hamiltonian / Riemannian Monte Carlo. See #104
• [ ] Standard metropolis hastings (non adaptive). See #116
• [ ] SMC. See #80, #120
• [ ] ABC. See #96, #56
• [ ] Particle MC. See #117
• [x] Nested sampling. See #107

[MichaelClerx]

As these have their own tickets, we can close this one!