Allow specifying xls_sheet

Currently, load_data.m contains

    if isglobalinbase('options_')
        % Check that the object is instantiated within a dynare session so that options_ global structure exists.
        % Should provide latter a mechanism to pass range and sheet to dseries constructor...
        range = evalin('base','options_.xls_range');
        sheet = evalin('base','options_.xls_sheet');
    else
        % By default only the (whole) first sheet is loaded.
        range = [];
        sheet = [];
    end

This does not work with makedataset in mom.run, because xls_sheet is a subfield of options_mom_, which is not even a global variable.