De-coupling well extension from lumping in MESS

[avcopan]

Via @lpratalimaffei:

with well_extension=True, the code activates the well extension options and also tries the lumping schemes. I think these two aspects should be decoupled.. in write_mess for well_extension=True, you write a mess input with well_extension for both basev1 and wextv1 and then you try the lumping scheme in wextv1 only. however, for run_mess/run_fits, well_extension=True will have the code search for the wextv1 input with the lumping scheme only