Validate directory/project structure before running a task

[rjpbonnal]

Some task can run only if a directory structure is satisfied, i.e. biongs convert:qseq:fastq:by_lane_index biongs convert:qseq:fastq:samples_by_lane

they expect to work in a directory with file and directories created from Illumina's demultiplexing software. demultipleixing creates multiple directories for each index 001, 002 and so on.