apptainer support

[sverhoeven]

The open source version of singularity has been renamed to apptainer https://apptainer.org/.

The eWaterCycle system should support this version.
Things that could/should be done:

• grpc4bmi should have BmiClientApptainer class
• setup-apptainer GH action and apptainer-versions repo (Singularity has been renamed to Apptainer setup-singularity#6)
• eWatercycle Python package should support apptainer
• snellius should have apptainer installed. This must be done by the SURF system administrators.
• App on SURF Research cloud should have apptainer installed
• DCache should have apptainer-images folder keep old dir name
• Replace any singularity usage in notebooks/documentation with apptainer Will be done in Update outdated docs #353

At the moment (24 jan 2022) the release of apptainer version 1.0.0 is imminent.

[sverhoeven]

Aptainer v1.0.1 has been released so this feature can be implemented now.

[sverhoeven]

For now the apptainer installation will symlink the singularity command to apptainer, so it is not that urgent to implement everything now.
Only when apptainer commands/arguments start to diverge from singularity we should have a apptainer support.

[sverhoeven]

The setup-apptainer GH action has been created. The tests in this repo use Docker, so no changes are needed in the CI jobs.

[sverhoeven]

Snellius has apptainer installed, see https://servicedesk.surf.nl/wiki/pages/viewpage.action?pageId=30660251#Apptainer(formerlySingularity)-Snellius

[sverhoeven]

In docs replace Singularity with Apptainer. Print deprecation warning when BmiClientSingularity is used.