Shanoir issue#2518

[alexpron]

This Pull Request aims at dealing with MAC OS added files within the BIDS data import. This PR solves issue #2518

MAC OS Hidden Files

There are several types of MAC Hidden files that are introduced either by Finder (and Zip) or by AppleDouble mechanism
see :

• DS_store.
• AppleDouble

Incompatibility with BIDS import

1. At the subject or session check level an file other than starting with sub- or sess- prefix was making the import crash
2. At data level these files if present in the datatype folder could be imported as real data 

Solution

• Filtering of the MACOS Hidden file to exclude them at the subject, session and data level
• Also added fix to deal with empty _sessions.tsv file that there making the process crash (dont know if its better now let me know)

[alexpron]

Note that I found a parsing error in the json metadata file when numbers are written into scientific mode (i.e. 2e^-12).
Online json parser declare the .json file used compliant