Correct plurality and add amount parameter

[edan-bainglass]

This PR corrects the naming convention w.r.t a few daemon endpoints that are clearly plural in nature. It also adds an amount query parameter to the incr/decr endpoints, mirroring the CLI.

Requires an update in aiida-rest-client. Will follow up once this is merged!

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.09%. Comparing base (2c174ed) to head (771b711).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #167   +/-   ##
=======================================
  Coverage   89.09%   89.09%           
=======================================
  Files          50       50           
  Lines        2036     2036           
=======================================
  Hits         1814     1814           
  Misses        222      222           

Flag	Coverage Δ
pytests	89.09% <100.00%> (ø)

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[Bud-Macaulay]

LGTM.

Glad you're even fixing the nits that i thought but didnt suggest! Mind-reader. :]

[edan-bainglass]

Glad you're even fixing the nits that i thought but didnt suggest! Mind-reader. :]

I can also pick nits 🙂