R2U2 C Execution API Hooks

[aaurandt]

With the R2U2 4.1 release, the following API hooks were removed from the C version (to eliminate possible confusion for end-user):

• r2u2_step executes a single R2U2 instruction. Good for cooperative multitasking
  - Would require changes to r2u2_engine_step
• r2u2_spin after the first spin, all signal values are latched but the time-step might not be complete. Most time-steps require two spins
  - Would require changes to r2u2_engine_step
• r2u2_tic runs until one time-step is complete. Default for most cases as it provides all verdicts available with current knowledge and ends when the signal vector should be refreshed
  - Now called r2u2_step in new API
• r2u2_run runs continuously, requires external refreshing of the signal vectors such as DMA or memory-mapped registers

These hooks may be beneficial to add back in.