Expose linear solver options

[mtanneau]

This PR makes it possible to customize the linear solver via solver parameters, instead of having to manually change the source code.

To make this possible, I introduced some trait-like options for linear solvers.
These options are exposed as solver parameters, which can be set by the user. The exact linear solver type if then dispatched on automatically.

Current options are

• Numerical type Tv, determined by the Model object
• Linear system to be solved: either AugmentedSystem or NormalEquations
• Linear solver backend. Currently supported are Lapack (for dense systems), Cholmod and LDLFact.

These options are now displayed at the beginning of the optimization, e.g.:

Linear solver options
  Precision    : Float64
  Backend      : CHOLMOD
  System       : Augmented system

See the documentation for more details and default options.

Finally, I added generic tests to simplify the testing of various linear solvers (customized ones in particular).