Rename isempty -> is_empty

[blegat]

Closes #505

[codecov-io]

Codecov Report

Merging #506 into master will increase coverage by 0.08%.
The diff coverage is 96.38%.

@@            Coverage Diff             @@
##           master     #506      +/-   ##
==========================================
+ Coverage   95.39%   95.47%   +0.08%     
==========================================
  Files          46       46              
  Lines        4694     5022     +328     
==========================================
+ Hits         4478     4795     +317     
- Misses        216      227      +11

Impacted Files	Coverage Δ
src/attributes.jl	96.66% <ø> (ø)	⬆️
src/Utilities/model.jl	98.36% <ø> (+0.07%)	⬆️
src/MathOptInterface.jl	0% <ø> (ø)	⬆️
src/Test/nlp.jl	0% <0%> (ø)	⬆️
src/Test/UnitTests/solve.jl	100% <100%> (ø)	⬆️
src/Utilities/mockoptimizer.jl	94.9% <100%> (ø)	⬆️
src/Utilities/universalfallback.jl	99.3% <100%> (ø)	⬆️
src/Test/intlinear.jl	99.44% <100%> (ø)	⬆️
src/Test/modellike.jl	100% <100%> (ø)	⬆️
src/Test/UnitTests/objectives.jl	99.09% <100%> (+0.49%)	⬆️
... and 11 more

---

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 6b237a3...4fc7467. Read the comment docs.