deco rewrite [was Re: Meaning of GF settings]

[Dirk Hohndel]

Robert,

I just pushed out a commit with the following commit message. Please
take a quick look to make sure that you are ok with the changes to the
comments at the top of the file and that I didn't mess anything up with
the one actual change that I made (described in the last paragraph of
the commit message).

Thanks

/D

    Clean up the rewritten deco.c
    
    In commit a7902f279a57 "Rewrite of the deco code" Robert kept the old code
    that he replaced around; I removed all that as it's in the git history if
    we ever need to look at it again but doesn't really help us in the file as
    it is.
    
    I also removed constants, variables and config parameters that aren't used
    in the new implementation and did some coding style / formatting changes
    to make deco.c more consistent with the rest of Subsurface. I also updated
    the comments at the top of the file to reflect reality.
    
    I did one change that actually affects the code. In the explanation of his
    changes Robert said that gf_low_pressure_this_dive is initialized to the
    exquivalent of 20m, yet his code added the surface pressure twice. I
    decided to change the default config value from 3 (bar) to 2 so that this
    indeed reflects (about) 20m (as in the code below surface_pressure is
    added to this value).