Add functions to internally compute correlations (integration)

[SantosRAC]

Add functions to internally compute correlations between OTU/ASV counts (transformed) and gene expression (transformed). After importing expression profiles and ASV/OTU counts, admins must be able to compute the correlations for pairs of gene - ASV/OTU.

Implementation involves:

• corALS (https://github.com/mgbckr/corals-lib-python) (initial Pearson correlation)
• Scipy (https://docs.scipy.org/doc/scipy/reference/generated/scipy.stats.pearsonr.html) (significance with permutation)

[SantosRAC]

UPDATES: for studies that have equal number of runs in the transcriptome and in the microbiome datasets/matrices, currently CORALS is being used to compute Pearson correlations between genes and microbes (OTUs). Adaptations will be made pretty soon to also accommodate correlations with ASVs.