Create singularity images from conda environments

[Thomieh73]

I used conda.yml files to create the conda environments from scratch .
That causes all kinds of installation issues, when trying it myself. Much better to generate singularity images of the environments.

[Thomieh73]

Installation of conda environments of

• multiqc (solved)
• microbecencus
• kraken

proved to be very difficult.
This needs to be solved soon.

[Thomieh73]

Conda environments are in the latest version of nextflow not enabled by default.

To use conda environments use:

conda.enabled = true

origin: https://www.nextflow.io/docs/latest/conda.html#enabling-conda-environment