[LORENE] New classes for general elliptic solver

[Philippe Grandclement]

Dear all,
    I am currently working on new classes for a more general elliptic 
solver. This solver should be able to solve an elliptic equation with 
both different variables and operators in each domain. This is currently 
under developement and not fully tested. So far we have the classes

    - change_var : describing a variable change in a given domain.
    - ope_elementary : an elementary elliptic operator. It can not be 
instanced and the derivative classes are :
            - ope_poisson
            - ope_helmholtz_minus
            - ope_helmholtz_plus
    - param_elliptic : an object containing all the operators and 
variable change. It is used by the new function Scalar::sol_elliptic


    All comments and suggestions are welcomed

Philippe Grandclement