Build/Extend test problem set

[yannikschaelte]

Implement a set of common ABC test and application problems with common API (prior, ground truth, data, model, ...)

• simple Gaussian of dim n with estimated mean and covariance
• multi-modal GMM
• gk distribution
• Lotka-Volterra
• Ricker model

and optionally various other SSA models

Benefit: Don't need to redo in publications.