prettifies code (space formatting in a slightly more consistent way)
some more code formatting; adds additional gpu wrapper functions
adds preprocessor function GPU_ERROR_CHECKING for ENABLE_VERY_SLOW_ERROR_CHECKING
Added the option to use an external source time function. The parameter EXTERNAL_SOURCE_TIME_FUNCTION has been set in constants.h.in. If this is set to true (it is false by default), a text file called stf is looked for in the data directory. This file should contain a normalized source time function, with the correct number of timesteps, which will be multiplied by the moment tensor at each timestep. Also, if the external stf is specified, the halfduration is set to 0 (not needed), and the simulation start time is set to 0 as well.
Changed print source time function back to false by default
adds GPU support for UNDO_ATTENUATION
Merge branch 'devel' of https://github.com/geodynamics/specfem3d_globe into devel