Add lazy_evaluation configuration option (default: on)
Add lazy_max_trace_length configuration option
Add support functions for trace management
Postpone the allocation of temporary Dats until data is accessed
Factorize 'par_loop' definition into base
Fix, ensure ParLoop::init is complete before evaluation in greedy evaluation
Merge pull request #223 from  OP2/laziness-mods

Fix problem of unbounded trace growth.  Adds options to turn lazy
evaluation off and limit the size of the trace.  Additionally, postpone
allocation of data on Dats until access.