Support for conda environments

In order to increase the number of people who will use rhea (and the citations we will get) I propose to support both singularity containers and conda environments. The user can decide whether to use singularity (--use-singularity) or conda (--use-conda) when running snakemake. This means that tests need to be run twice which should not be a problem. The majority of the tools are available in the bioconda channel. We will only need to add our custom tools (e.g. TIN score calculation in bioconda). Let me know what you think.