Add some set operations

[eeshan9815]

No description provided.

[coveralls]

Coverage increased (+0.03%) to 92.277% when pulling a675619 on eeshan9815:set-operations into 777ad9d on JuliaIntervals:master.

[codecov-io]

Codecov Report

Merging #154 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #154      +/-   ##
==========================================
+ Coverage   92.24%   92.27%   +0.03%     
==========================================
  Files          21       21              
  Lines        1006     1010       +4     
==========================================
+ Hits          928      932       +4     
  Misses         78       78

Impacted Files	Coverage Δ
src/IntervalArithmetic.jl	100% <ø> (ø)	⬆️
src/intervals/set_operations.jl	100% <100%> (ø)	⬆️

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[lbenet]

Thanks for the contribution!

I think the definition of ⊂ corresponds to ⪽ (isinterior); can you confirm? (If I recall correctly, the reason we used that name for the function and the symbol is because they are used like that in the standard.) In any case, the "other-way-around" operators are certainly a good addition.

[dpsanders]

Thanks!