Throw error when a snapshot with same id was created multiple time

[lukfor]

Throws an error when a user creates more than one unnamed snapshot or mutiple snapshots with the same id to avoid unexpected snapshot matching. Fixes #158

[RHReynolds]

@lukfor, is there a way we can allow for deliberate snapshot matching?

I've actually found it very useful to be able to have multiple tests refer to the same snapshot id. When testing a pipeline's logic, there will be times when skipping a process will impact files downstream of the process but not upstream. In this situation, it's been quite useful to group files that are expected to be unchanged into one snapshot id (which can be used across multiple tests) and those you expect to change. E.g.

{ assert snapshot(
                    path("$outputDir/cat/SAMPLE3_SE.merged.fastq.gz"),
                    path("$outputDir/pipeline_info/samplesheet.valid.csv")
                    ).match("common_files") 
                }

With the new modification, if I now want to check that the files I expect to be unchanged are truly unchanged across multiple test, I would have to manually compare md5s in the snapshot.