Use the AlgContext  guard constructor without EventContext for single thread

Closes #73 and #72

See merge request gaudi/Gaudi!937

Closes #71

see #73

A variable hook `GENREFLEX_JOB_POOL` is added,
that allows limiting the parallelization of genreflex commands
independently of the default parallelism when using ninja.
See the ninja [manual](https://ninja-build.org/manual.html#ref_pool)
for more details on job pools. This is especially needed
when distributing the compilation as the number of jobs is very high
and trying too many genreflex calls locally can use up the memory.

The new variable can be set from the toolchain or the pre-loaded initial
cache.

Charles Leggett authored 5 years ago and Marco Clemencic committed 5 years ago

This reverts merge requests !869 and !932

Closes #76