Liu 247

[awicenec]

Main objective was to support *args and **kwargs for PyfuncApps. In addition to that we are now also supporting positional only args and kwargs only. Thus we are covering all Python cases. Additional changes:

• Fixed the issue with the Travis version of doxygen wrongly extracting information from the docstring, if that contained highlighting markup. That messed up the hover help text of a number of components, including the updated PyfuncApp. The fix involved moving the Travis tests to Ubuntu Focal. Also cleaned up the default palettes extracted by xml2palette and enabled the extraction of a few more of the existing apps.
• added a dummy PythonApp to simple.py to trigger the extraction of the associated palette documentation. The PythonApp palette component only existed as a hard-coded component in EAGLE.
• Aligned the EnvironmentApp category with EAGLE, where this was called EnvironmentVariables.

[coveralls]

Coverage increased (+0.8%) to 81.193% when pulling a310776 on liu-247 into 3068a2f on master.

[rtobar]

I just left some minor comments, but those are not too important.

What I find more substantial is that most of the new code in pyfunc.py is not tested:
https://coveralls.io/builds/48860216/source?filename=daliuge-engine%2Fdlg%2Fapps%2Fpyfunc.py. I'm guessing some manual tests were done, but it'd be great to see unit tests for these new bits, as otherwise we'll quickly derail when new changes come in.

[rtobar]

It's not crystal clear what the idea of this new PythonApp is, given it's almost identical to SleepApp except for the name and for not having the sleepTime parameter. If it's here to stay let's at least add a test for it just to ensure it's actually correct (it does look correct though).

[awicenec]

As mentioned in the original comment for this PR the PythonApp is required to enable people to develop a graph and a palette from scratch in EAGLE. It always existed for that reason, but was part of the hard-coded palette in EAGLE. Since a PythonApp is in some way such a fundamental component in DALiuGE, there was no class with that name in the code and thus this was the only normal component missing from the EAGLE hard-coded palette. This class only exists to enable the automatic creation of the associated component in the Raw Component palette. Since the appclass parameter in the EAGLE doxygen section is actually empty, the code will in fact never be called (although it is functional). Thus we could go a step further and just put a 'pass' into the code body.