Update to conda environments

This merge request changes the following:

1. Split conda.yaml in conda_linux.yml and conda_macos.yml. The reason for this is that having BUSCO v5.2.2 as a dependency does not work on macOS.
2. Add ASTER v1.3 as dependency to conda instead of having it as a submodule. Also update error messages in the code accordingly.

Still to do (can be done during review process):

• Update all documentation that has to do with the conda.yaml file, BUSCO and ASTER.
• Add message to functions that call busco that one cannot run this on macOS.
• Fully remove git submodule. Since this can be quite messy, I would like to do this as the last step in this merge request. The necessary steps are: git submodule deinit -f ASTER; rm -rf .git/modules/ASTER; git rm -f ASTER.
• Update CHANGELOG.md