[refactor] modules refactor

[fedemagnani]

Modules should have a rigorous hierarchy reflecting the family solvers

Modules
├── First-Order
│   ├── Gradient / Subgradient
│   ├── Accelerated
│   ├── Coordinate Descent
│   └── Mirror Descent
├── Second-Order
│   ├── Newton
│   ├── Quasi-Newton
│   └── Trust Region
├── Conjugate Gradient
├── Interior Point
├── Subgradient & Bundle
├── Proximal & Splitting
│   ├── Proximal Point
│   ├── ADMM
│   └── Douglas-Rachford
├── Cutting Plane & Ellipsoid
├── Stochastic
│   ├── SGD
│   └── Variance Reduction
├── Frank-Wolfe
├── Primal-Dual
└── Specialized Structured Methods