Install Ascat with bioconda

[maxulysse]

• Install ASCAT 2.5.2 with conda in the environment.yml file
• Changed r-base from 3.5.1 to 3.4.1 and bioconductor-rtracklayer from 1.42.1 to 1.38.3 to satisfy conda dependencies when adding ASCAT to the recipe

@malinlarsson is working on updating ASCAT, so I'm adding it to the base docker container so we can have a clean conda install with all tools.

Many thanks to contributing to nf-core/sarek!

Please fill in the appropriate checklist below (delete whatever is not relevant). These are the most common things requested on pull requests (PRs).

PR checklist

• This comment contains a description of changes (with reason)
• If you've fixed a bug or added code that should be tested, add tests!
• If necessary, also make a PR on the nf-core/sarek branch on the nf-core/test-datasets repo
• Ensure the test suite passes (nextflow run . -profile test,docker).
• Make sure your code lints (nf-core lint .).
• Documentation in docs is updated
• CHANGELOG.md is updated
• README.md is updated

Learn more about contributing: https://github.com/nf-core/sarek/tree/master/.github/CONTRIBUTING.md