major hdds update to FDC,CDC

[Richard Jones <richard.t.jones@uconn.edu>]

I have taken the new materials specifications for the CDC and FDC from
gluex-doc-805 and implemented the changes in hdds.  The modified
geometry is now the latest release in subversion.  The following
changes have been made.

1. Reduce the FDC cathode plane Kapton thickness from 50 to 25 microns.
2. Reduce the FDC cathode strip copper layer from 5 microns to 1 micron.
3. Remove rohacell backing on cathode strip planes in the active region.  This also gets rid of the two layers of epoxy that bond the rohacell to the Kapton on one side, and the mylar on the other.
   
4. Reduce the mylar film thickness from 25 microns to 5 microns with 0.5 microns of aluminum on one side.
   
5. Change chamber gas mixture to 40/60 Ar/C02.
6. New heterostructure for FDC support frames, as per gluex-doc-800.
7. Replace CDC aluminum downstream plate with carbon fiber / epoxy composite.
8. Reduce the CDC downstream plate from 6mm to 4mm thickness.

Can someone confirm that the mylar is really supposed to be 5 microns? 
I inferred this value from the material budget in gluex-doc-805 table 2
and want to make sure it was not just a mistake.

Note that the modified CDC layer structure with additional inner layers
and a smaller outer radius than 60cm  is NOT a part of the new
release.  Those modifications were introduced to hdds back in March for
the purpose of exploring background rates.  However as they were
originally designed the modified inner layer structure overlaps with
the target geometry and so cannot be merged into the default geometry
as of yet.  Those changes have been rolled back by commenting out the
relevant sections in CentralDC_HDDS.xml.

Richard Jones

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