Adding steady thermal element QUADTH

[dmitryikh]

1. Implementation of QUADTH thermal element with SurfaceLINETH surface BC (heat flux, convection)

2. add QUADTH test (simple 2d case with convection)

3. SpeedUp compilation time.

   • Add and use cotire.cmake module to use precompiled headers
   • make will not build test executables. Now you need to run make check in order to compile and run FUNC tests

4. some cleanup in DofCollection class

5. now DofCollection track set of unique DofTypes in it

6. VtkProcessor will output point and cell data based on used unique DofTypes provided by FEStorage

7. rename FEStorage->addForce to addDofLoad

8. Improve travis job:

   • add to travis.yml: dist: trusty
   • add make -j 2 to travis to speedup compilation