[bugfix] Support inheritance in pipeline hooks

[vkarak]

See documentation for more details + unit tests.

Fixes #991.

[codecov-io]

Codecov Report

Merging #994 into master will increase coverage by 0.02%.
The diff coverage is 96.61%.

@@            Coverage Diff             @@
##           master     #994      +/-   ##
==========================================
+ Coverage   91.82%   91.84%   +0.02%     
==========================================
  Files          80       80              
  Lines       10674    10732      +58     
==========================================
+ Hits         9801     9857      +56     
- Misses        873      875       +2

Impacted Files	Coverage Δ
reframe/core/pipeline.py	92.22% <100%> (+0.17%)	⬆️
unittests/test_pipeline.py	94.36% <95.74%> (+0.1%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update dc59627...7f9d1c7. Read the comment docs.