Expose the gas measurements

[pirapira]

This PR adds an API gas_measurements(contracts_version) that exposes the gas measurements as a dictionary.

[codecov]

Codecov Report

Merging #1027 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1027      +/-   ##
==========================================
+ Coverage   85.14%   85.18%   +0.04%     
==========================================
  Files          20       20              
  Lines        1373     1377       +4     
  Branches      182      182              
==========================================
+ Hits         1169     1173       +4     
  Misses        158      158              
  Partials       46       46

Impacted Files	Coverage Δ
raiden_contracts/contract_manager.py	97.65% <100%> (+0.07%)	⬆️

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[codecov]

Codecov Report

Merging #1027 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1027      +/-   ##
==========================================
+ Coverage   85.14%   85.18%   +0.04%     
==========================================
  Files          20       20              
  Lines        1373     1377       +4     
  Branches      182      182              
==========================================
+ Hits         1169     1173       +4     
  Misses        158      158              
  Partials       46       46

Impacted Files	Coverage Δ
raiden_contracts/contract_manager.py	97.65% <100%> (+0.07%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3800d77...1ebec79. Read the comment docs.

[rakanalh]

LGTM