Add gram_operate and refactor switchedsystem test

[blegat]

It also fix a bug with bridges not being added for [D]SOS variables

[coveralls]

Coverage increased (+3.0%) to 78.924% when pulling 99cbf4d on bl/gram_operate into 991c1a3 on master.

[codecov-io]

Codecov Report

Merging #105 into master will increase coverage by 2.81%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #105      +/-   ##
==========================================
+ Coverage   78.42%   81.23%   +2.81%     
==========================================
  Files          22       22              
  Lines         496      501       +5     
==========================================
+ Hits          389      407      +18     
+ Misses        107       94      -13

Impacted Files	Coverage Δ
src/variable.jl	60.71% <100%> (+4.71%)	⬆️
src/gram_matrix.jl	85.71% <100%> (+0.8%)	⬆️
src/sosdec.jl	91.66% <0%> (+34.9%)	⬆️

---

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 991c1a3...99cbf4d. Read the comment docs.