Functions to define stages and their constituent steps.
The macros combine several steps and assign them to relevant
stages.
See dsl_get() for functions to access the storage for the stages
and their steps.
get_stage() returns a TicStage object for a stage given by name.
This function can be called directly in the tic.R configuration file,
which is processed by dsl_load().
add_step() adds a step to a stage, see step_hello_world()
and the links therein for available steps.
add_code_step() is a shortcut for add_step(step_run_code(...)).