Use parameter file instead of base64 encoded variable

[Gregor Sturm]

Instead of using an obscure base64 encoded string that is injected into the bash script, I should write all parameters to a yml (or json file) that is written to the work directory (e.g. .nxfvars.yml). This has several advantages

• the nxfVars(task) command can be executed outside the script block and does not need to use the weird GString injection
• The config file is much more transparent, which facilitates debugging
• It opens up another avenue for executing code outside nextflow: a parameter config file can be used