Add GRN support

[rvinas]

This PR constructs GRNs networks by downloading data from public databases. Currently, it supports TRRUST and RegNetwork, but others can be added in the future. I tried to follow the coding conventions from PPI networks. It would be great to double-check the following:

• [x] Whether the download directory is appropriate. I'm storing the data in datasets/<name of database>.
• [x] How the root directory is retrieved. I am using a relative path, e.g. root_dir = os.path.dirname(os.path.realpath("../")). Perhaps there's a better way to achieve this?
• [x] For RegNetwork, I am not entirely sure what the regulatory interaction types -p and -/- mean exactly. I am mapping them to the unknown class ?.

[a-r-j]

Hey @rvinas will review this now. I had a look at the docs for RegNetwork. I think -/- means the directionality is unknown. I can't make sense of -p and it happens infrequently enough that it's probably safe to assign to unknown.

[codecov-io]

Codecov Report

Merging #48 (a223817) into graphein-api (6c946d9) will decrease coverage by 1.89%. The diff coverage is 0.43%.

@@               Coverage Diff                @@
##           graphein-api      #48      +/-   ##
================================================
- Coverage         19.33%   17.43%   -1.90%     
================================================
  Files                25       33       +8     
  Lines              1583     1870     +287     
================================================
+ Hits                306      326      +20     
- Misses             1277     1544     +267     

Impacted Files	Coverage Δ
graphein/grn/config.py	0.00% <0.00%> (ø)
graphein/grn/edges.py	0.00% <0.00%> (ø)
graphein/grn/features/node_features.py	0.00% <0.00%> (ø)
graphein/grn/graphs.py	0.00% <0.00%> (ø)
graphein/grn/parse_regnetwork.py	0.00% <0.00%> (ø)
graphein/grn/parse_trrust.py	0.00% <0.00%> (ø)
graphein/utils.py	31.03% <25.00%> (ø)
... and 5 more

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