Implement OTU (model) and associated methods

[SantosRAC]

OTU tables (counts) will be permitted in CoNekT Grasses Microbiome.

Main models in mind:

• OperationalTaxonomicUnitMethod - will describe methods used for generation of OTUs (e.g., clustering algorithms, threshold, reference database if open or closed reference method is used, reference db release to enable reproducible analyses). It will also link to available literature, if available (e.g., Wallace et al. 2018 is the first to be included with OTUs).
• OperationalTaxonomicUnit - will mainly store the representative sequence for the OTU, defined by method (OperationalTaxonomicUnitMethod above)

Models and methods will serve as basis for implementation of further functions (e.g., generate/import profiles, OTU classification, associations/correlations between transcriptome and microbiome).

• ⚠ Importantly, using OTUs will only be enabled in studies comprising a single literature!

[SantosRAC]

UPDATES: classes for OTUs and associated methods were implemented for initial correlations and showing on OTU and Study pages, as well as for showing results of profiles and correlations in the front-end.