checkExecutability(): adapter file for trimming exists

[vinjana]

Currently, the file defined by the CLIP_INDEX variable is checked, but only if it does not start with $DIR_EXECUTION. The file should always be checked and the workflow cancelled, if the file is not readable. The check should already be done in QCPipeline, as clipping can be done with all alignment data (currently in BisulfiteCoreWorkflow)

Use ExecutionContext.getExecutionDirectory for this. Alternatively, it may be sufficient to substitute the $DIR_EXECUTION by ${DIR_EXECUTION}.