Make ProcessBuilder overridable and add helper for WorkerRuns with costs

Part of enterprise#55

changed milestone to %Arkindex 1.8.0

requested review from @babadie

assigned to @erouchet

Why not reuse the same approach as for ponos methods:

• still use import_string
• but only on specific methods, not on the whole class

As I see in the related enterprise MR, this is only superseeding 2 method calls in ProcessBuilder: so you could extend just these functions ?

resolved all threads

added 4 commits

• 1ce76a0e...b8c174de - 2 commits from branch release-1.8.0
• 605fa702 - Make ProcessBuilder overridable in EE
• d8f1fecd - Add a has_costs helper

Compare with previous version

resolved all threads

merged