softwareVersion should be optional unconditionally

[adk-swisstopo]

The softwareVersion key is marked as "optional" with the following note:

The key can be omitted if the software is currently in initial development and has never been released yet.

The easiest way to implement publiccode.yml for software projects is to check a static version within their source code repository. Ensuring the softwareVersion field is up to date is painful (either that's extra release toil or more automation to build) and of questionable value. I expect many projects won't bother. We certainly don't.

Searching through github, I find 920 files matching path:/publiccode.yml$/, only 760 of which have a softwareVersion key. I don't know how reliable these numbers are as github search is obviously fuzzy. Here are random examples of projects that have tagged releases without a softwareVersion key:
https://github.com/CISOfy/lynis
https://github.com/CatalaLang/catala/
https://github.com/InseeFrLab/onyxia/

For those that do specify the softwareVersion field, I expect it's often out of date. For example:
https://github.com/OS2iot/OS2iot-docs/
https://github.com/torakiki/pdfsam/
https://github.com/ghtmtt/DataPlotly

I suggest one of the following two changes:

• let the softwareVersion field be optional regardless of the release status of the project;
• remove the softwareVersion field.

[bfabio]

@adk-swisstopo nice catch.

softwareVersion is optional and that sentence is kinda meant as a "for instance, in this case you can skip it", not as "otherwise it's required". The wording is confusing and makes it look conditional when it isn't.

We should fix the text to make that clear, I'll open an issue/PR.

The parser already does the right thing (treats it as optional).