Issue #182 add namespace support to DataCube.process and related

[soxofaan]

fixes #182: adds namespace support to DataCube.process, PGNode, ProcessGraphVisitor, GraphFlattener

This improvements changes the ProcessGraphVisitor API changed a bit by adding namespace argument to enterProcess and leaveProcess (which will break openeo-python-driver and openeo-geopyspark-driver)
Before pushing this, I wanted to go through PR+review first

(I bumped version to 0.5.0a1 to clearly mark API change)

[soxofaan]

related PRs for python and geopyspark driver:

• Update to ProcessGraphVisitor API change (namespace support) openeo-python-driver#61
• Update to ProcessGraphVisitor API change (namespace support) openeo-geopyspark-driver#67

[soxofaan]

@jdries can you have a quick look if it's ok to merge these PRs?

[jdries]

Reviewed, the most critical question to me is whether 'namespace' is accepted by openEO partners as a new word in our terminology? We have been using it, but not sure if it was generally presented in the PSC.
@m-mohr Do you happen to know?
Of course, it's not like I immediately have an alternative available.

[soxofaan]

"namespace" as a concept and field is already part of the API, e.g. see the <ProcessNode> definition at https://openeo.org/documentation/1.0/developers/api/reference.html#section/Processes/Process-Graphs

it was introduced here: Open-EO/openeo-api#305

[m-mohr]

Yes, that seems all fine. We don't have agreed on Open-EO/openeo-api#348 yet though.

[soxofaan]

We don't have agreed on Open-EO/openeo-api#348 yet though.

indeed, but that is not a blocker for this issue #183:
it just introduces function arguments to allow setting the (optional) namespace field in process nodes (following the existing 1.0.0 spec)

[soxofaan]

thanks!