Add abstract matrix sets

[blegat]

Other matrix sets are defined in SumOfSquares:
https://github.com/JuliaOpt/SumOfSquares.jl/blob/master/src/diagonally_dominant.jl
https://github.com/JuliaOpt/SumOfSquares.jl/blob/master/src/psd2x2.jl
https://github.com/JuliaOpt/SumOfSquares.jl/blob/master/src/empty_bridge.jl
and having abstract sets would allow these sets to benefit from what is implemented for the PSDSquare and PSDTriangle sets such as the SquareBridge.

[codecov-io]

Codecov Report

Merging #731 into master will decrease coverage by 0.01%.
The diff coverage is 94.73%.

@@            Coverage Diff             @@
##           master     #731      +/-   ##
==========================================
- Coverage   94.21%   94.19%   -0.02%     
==========================================
  Files          54       54              
  Lines        5821     5825       +4     
==========================================
+ Hits         5484     5487       +3     
- Misses        337      338       +1

Impacted Files	Coverage Δ
src/Bridges/square_bridge.jl	98.27% <100%> (ø)
src/Utilities/results.jl	90.26% <100%> (ø)	⬆️
src/Bridges/Bridges.jl	100% <100%> (ø)	⬆️
src/sets.jl	92.72% <85.71%> (-1.4%)	⬇️

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