Initial .gitignore file
Fixes #7 - parallel CMake build

Identical file names in multiple directories were causing file
overwrites during parallel CMake build.  Fixed this by splitting the
build into two subdirectories.
Readd mysteriously disappearing file
Bugfix in 1d model output

 - Value UNITS was not written out in model_discontinuities.f90
 - number of columns was wrong in background_models.F90
Merge branch 'master' of https://github.com/geodynamics/axisem
Merge branch 'master' of https://github.com/geodynamics/axisem
Update .gitignore
Manual: detailed description of external models
formatting of error msg