added complex sum to the function _linear_expansion

[Bumblebee00]

for issue 1199

[ChrisRackauckas]

This looks right to me. Needs tests.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 20.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 12.99%. Comparing base (9df148a) to head (1da6587).
⚠️ Report is 1102 commits behind head on master.

Files with missing lines	Patch %	Lines
src/linear_algebra.jl	20.00%	4 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1492      +/-   ##
==========================================
+ Coverage    8.79%   12.99%   +4.19%     
==========================================
  Files          51       51              
  Lines        4924     4957      +33     
==========================================
+ Hits          433      644     +211     
+ Misses       4491     4313     -178     

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[Bumblebee00]

okay so I am adding some tests, but I encounter strange problems. For example I wanted to add:

@testset "linear_expansion with imaginary numbers" begin
    @variables x y
    a, b, islin = Symbolics.linear_expansion(1im*x + 1im*y, x)
    @test islin && isequal(a, 1im) && isequal(b, 1im*y)
end  

but srangely this test fails for b. even tough printing b gives (0 + 1im)*y, this is considered different from 1im*y. also valuating isequal(b, (0 + 1im)*y returns false.

Does anybody can tell me why this happens and how to check if b is equal to 1im*y ?