sra2isa conversion: add support for SRA Pool/Pool_Member representation

[proccaserra]

reported by John Matese at Princeton when converting SRP068846 http://www.ebi.ac.uk/ena/data/view/SRX1544187

also, the representation pattern used in this project departs from the one supported, which relies on defining 'multiplex identifier' as defined by MIXS checklists and supported by the corresponding ISA configuration for metagenomics studies and for targeted gene surveys.