build: improve version consistency in environment ymls

[ninsch3000]

In order to avoid packages specified in the environment.yml being updated upon using environment-dev.yml on top, all packages with their versions should be copied over from environment to environment-dev yml.
Furthermore, for ease-of-use, the option of installing singularity inside the scrnasim conda environment should be given.

We should then have four environment.yml files:

• environment
• environment.root, containing singularity
• environment.dev, containing all packages for dev PLUS a copy of all packages in environment
• environment.root.dev, containing all packages for dev PLUS a copy of all packages in environment.root

Don't forget to update the README accordingly