Documentation: advanced concepts of workchain design

[sphuber]

Fixes #1130 and #1138

Wrote extensive documentation on all the features of the WorkChain
and how to write robust, maintainable and modular workchains. Described
all the features of the ProcessSpec with the newly added Ports and
PortNamespaces, the context and how to add futures, the reporting system
and how to abort workchains and set their finish status.

[codecov-io]

Codecov Report

Merging #1459 into workflows will decrease coverage by 8.91%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##           workflows    #1459      +/-   ##
=============================================
- Coverage      56.74%   47.82%   -8.92%     
=============================================
  Files            277      277              
  Lines          34046    34046              
=============================================
- Hits           19319    16283    -3036     
- Misses         14727    17763    +3036

Impacted Files	Coverage Δ
aiida/backends/djsite/db/subtests/nodes.py	0% <0%> (-100%)	⬇️
.../orm/implementation/django/calculation/__init__.py	0% <0%> (-100%)	⬇️
aiida/backends/djsite/settings/__init__.py	0% <0%> (-100%)	⬇️
aiida/backends/djsite/db/subtests/query.py	0% <0%> (-100%)	⬇️
...da/backends/djsite/db/subtests/djangomigrations.py	0% <0%> (-100%)	⬇️
aiida/orm/implementation/django/backend.py	0% <0%> (-100%)	⬇️
aiida/backends/djsite/db/subtests/generic.py	0% <0%> (-100%)	⬇️
aiida/orm/implementation/django/log.py	0% <0%> (-96.3%)	⬇️
aiida/orm/implementation/django/code.py	0% <0%> (-95%)	⬇️
.../implementation/django/calculation/job/__init__.py	0% <0%> (-91.31%)	⬇️
... and 36 more

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