Add JuMP-like API

[joaquimg]

closes #271

• remove slack data from conic model
• improve parameters check OR allow mixing parameters and function (when setting sensibs)
• fix constraint names on copy to diff (see test/jump_wrapper.jl)
• cleanup support for constrained variables
• cleanup error messages from parameters.jl

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.44%. Comparing base (93f058d) to head (3cea22f).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #281      +/-   ##
==========================================
+ Coverage   89.08%   89.44%   +0.36%     
==========================================
  Files          15       16       +1     
  Lines        1969     1999      +30     
==========================================
+ Hits         1754     1788      +34     
+ Misses        215      211       -4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.