Making a src directory for shared code

[lfelberg]

To make a shared directory for code that is identical between models:

• [x] Find files that are completely identical between solvers
  • [x] BesselCalc.*
  • [x] Constants.*
  • [x] MyMatrix.h
  • [x] ReExpCalc.*
  • [x] SHCalc.*
  • [x] util.h
• [x] Move them to a separate directory in pb_solvers
• [x] Remove the shared files and their tests from pb[s]am
• [x] Change CMake for PBAM and PBSAM
• [x] Find a way to deal with files that have slight differences

[lfelberg]

Done all but the last step... big difference files (system) are kept separate.