Implement CalcJob process class

[sphuber]

Fixes #2377, fixed #2280, fixes #2219 and fixed #2381

This commit can be summarized in three steps:

• Reimplementation of a job calculation as Process called CalcJob
• Changing job calculation to be purely informational
• Remove the old job calculation mechanics and business logic

The old way of creating a job calculation, was to subclass the
JobCalculation class and override the _use_methods class method to
define the input nodes and the _prepare_for_submission to setup the
input files for the calculation. The problem was that these methods were
implemented on the Node class, thus mixing the responsabilities of
running and introspecting the results of a completed calculation.

Here we define the CalcJob class, a subclass of Process. This class
replaces the old JobCalculation and allows a user to defined the
inputs and outputs through the ProcessSpec, just as they would do for
a WorkChain. Except, instead of defining an outline, one should
implement the prepare_for_submission, which fulfills the exact same
function as before, only it is now a public method of the CalcJob
process class.

Finally, the role of the job calculation state, stored as an attribute
with the key state on the CalcJobNode has changed significantly.
The original job calculations had a calculation state that controlled
the logic during its lifetime. This was already superceded a long time
ago by the process wrapper that now fully governs the progression of the
calculation. Despite the calculation state no longer being authoritative
during the calculation's lifetime, it was still present. Here we finally
fully remove and only leave a stripped down version. The remaining state
is stored as an attribute and is a sub state while the CalcJob process
is in an active state and serves as a more granual state that can be
queried for. This is useful, because the process status, which also
keeps similar information is human readable and doesn't allow for easy
querying.

[coveralls]

Coverage increased (+0.8%) to 69.503% when pulling 70f5ff6 on sphuber:fix_2377_implement_calc_job_process into de14357 on aiidateam:provenance_redesign.

[giovannipizzi]

Just a couple minor things

[giovannipizzi]

A few notes here

• you should escape the underscore (should be \_)
• maybe better to do something like (attribute.key='environment_variables' OR attribute.key like 'environment\_variables.%')

Similar for the others where applicable (where it's a single variable and not a dict/list, you don't need to do like instead, just =)

[giovannipizzi]

Don't we want to still do some validation depending on the (process_)state? Or it's somewhere else?

[giovannipizzi]

Wonderful!