WIP

[skip ci]

 + New routine get_user_defined_boundaries()

 + Add @dprior method for plotting prior densities.

 - check_mode_file() signature change (do not pass options_ as an
   argument, remove bayestopt_), bug fixes (wrong indexing of cell
   arrays) and add unit tests.

 + set of private functions under the estimation subfolder (code factorization).

 + initialguess() returns the initial guess for the estimated parameters

 + @dprior/isempty() method, return true iff object is empty

 + derror() Throws an error without the stack trace under
   MATLAB. Behaviour unchanged under Octave. Ideally,we should print
   the stack trace when in debug mode. Can we test if we are in debug
   mode without passing options_?

 + isbayesian() returns tre iff bayesian approach is considered.

 - set_prior returns a dprior object.

 - set_prior ne devrait en principe pas être appelé quand on ne
 considère pas une approche bayésienne... C'est pourtant le cas car
 cette fonction retourne aussi la condition initiale et les bornes de
 l'optimisateur. Il faut casser cette fonction.

uniform_specification() was throwing an error if the uniform prior is
defined with the expectation and variance.