Qmm

Changed
- Docstring corrections.
- Inverse covariance fix.
- Constant computation fix in QuadObjective.

Changed
- Fix `fun` calc in linear CG (lcg)
- Fix `fun` attr in OptimizeResult
- Clean OptimizeResult to remove unused fields
- Fix step for real to complex operators
- Use abs() before square to avoid complex casting
- Fix sectioning in documentation

Changed
- Refactor '_vect' as 'vect' and add 'vectorize' decorator.
- Refactor 'metric' array to 'invcov' as callable.
- Update demo and image illustration.
- Fix pyproject.toml for pylint.

Added
- Add preconditioner to 3MG.

Changed
- Better typing and some typo correction.

Added
- Add *MixedObjective* class, a list-like object that represent the sum of
objectives. Support "+" operartor.
- *Objective* support "+" operator and returns a *MixedObjective*.
- *BaseObjective* have *lastv* attribute that return the last computed objective
value.
- *BaseObjective* have a *name* string attribute.
- Add a CHANGELOG.md file.

Changed
- rename *lastv* to *lastgv*.
- rename *calc_objv* to *calc_fun*
- rename *init* to *x0* like in scipy.

Added
- add *lastv* attribut to *BaseObjv* that equals to the objective value after
last gradient evaluation.
- add *calc_objv* flag to compute criterion value with low overhead.
- add a *callback* function to optimization algorithms.

Changed
- rename *Criterion* to *Objective*.
- rename *Potential* to *Loss*.