Implement DESIGN_MATRIX config keyword

[xjules]

The main goal of this issue is to specify an excel sheet (.xlsl) file containing the sensitivity analysis parameters.
The argument list for DESIGN_MATRIX should include

• the path (to the excel sheet file)
• name of the sheet with the actual design parameters (design-sheet)
• name of the sheet with the default values (default-sheet)

An initial work has been already done by @oyvindeide #8099

[xjules]

WIP: man sketch

DESIGN MATRIX

.. design_matrix:

DESIGN_MATRIX is meant used to read, validate and internalize into ert storage a design matrix given in XLSX-format.
:code:DESING_MATRIX supports from 1 to 3 positional arguments:

• XLSX file containing design and default sheet
• the name of the design sheet (defaults to DesignSheet)
• the name of the default sheet (defaults to DefaultSheet)

Ex: DESIGN_MATRIX poly_design.xlsx DESIGN_SHEET:DesignSheet01 DEFAULT_SHEET:DefaultSheet

The combination with GEN_KW parameters is supported; nevertheless in case of overlapping names, ert requires the full overlap of GEN_KW parameter group with the parameters located in the design parameters otherwise it will yield a validation error. In case of the full overlap the group name will be acquired from the GEN_KW, but the actual parameter values will be read from the design matrix.