adding add_group

[mmaelicke]

resolves #30

add API:

• [x] add_group
• [x] add_project
• [x] Entry.make_composite()
• [ ] add e2e-test (and change the existing group test)
• [x] docs

[codecov[bot]]

Codecov Report

Merging #99 into master will decrease coverage by 0.26%. The diff coverage is 36.84%.

@@            Coverage Diff             @@
##           master      #99      +/-   ##
==========================================
- Coverage   49.79%   49.52%   -0.27%     
==========================================
  Files          52       52              
  Lines        2161     2199      +38     
==========================================
+ Hits         1076     1089      +13     
- Misses       1085     1110      +25     

Flag	Coverage Δ
#e2e	49.52% <36.84%> (-0.27%)	:arrow_down:

Impacted Files	Coverage Δ
metacatalog/api/__init__.py	100.00% <ø> (ø)
metacatalog/api/_mapping.py	100.00% <ø> (ø)
metacatalog/api/find.py	58.65% <ø> (-0.56%)	:arrow_down:
metacatalog/models/entrygroup.py	70.73% <0.00%> (-1.77%)	:arrow_down:
metacatalog/api/add.py	38.88% <10.00%> (-3.25%)	:arrow_down:
metacatalog/models/entry.py	63.71% <70.58%> (+0.55%)	:arrow_up:

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