Parameterization of the gas properties - Usage in the MC

Parameterization of the gas properties - Usage in the MC
for task:
 https://trello.com/c/cLXI4aij/2-code-parameterized-bb-formula-data-mc-correspondence

A       Sim/AliTPCGen.h       - New file. "Potential" acces to the Geant3 common block
M       Sim/AliTPCv2.cxx      - usage of the parameterizaed gas parameters + max. number of steps 30000 ->120000
M       Sim/AliTPCv4.cxx      - ...
M       Sim/AliTPCv2.h        - include Geant3 common block acces
M       Sim/AliTPCv4.h        - ...
M       Sim/AliTPC.cxx        - changed definitions of gas mixtures

M       Rec/AliTPCseed.cxx    -  correct for different mean energy loss of the differnt gas compositions