Add option do deactivate sanity check for reading sample sheets and adding samples

[alexblaessle]

For illumina sample sheets with 10X multiplex libraries we often have multiple samples with the sample indexes. This obviously does not work with the package right now. Another option would be to replace all errors with warnings, as done in this PR: https://github.com/clintval/sample-sheet/pull/117

Would it be possible to add this or simply merge this PR? I am happy to help.