Implement vcat(::AbstractBandedMatrix...)

[max-vassili3v]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.93%. Comparing base (47c15ab) to head (9e8706b).
Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #448      +/-   ##
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+ Coverage   89.61%   89.93%   +0.32%     
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  Files          25       25              
  Lines        3571     3675     +104     
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+ Hits         3200     3305     +105     
+ Misses        371      370       -1     

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[max-vassili3v]

I've had a lot of issues involving randomly generated banded matrices where one or both of the bandwidths exceed the dimensions of a matrix (e.g a 2x2 BandedMatrix with bandwidths (4,3)). Is it worth adding something in the constructors that clamps l and u between -m and n-1 and -n and m-1 respectively?

[dlfivefifty]

No because having more bands than the dimension is permitted

I would recommend not using random bandwidths in the unit tests

[dlfivefifty]

Suggested change

	LinearAlgebra.vcat(x::Union{OneElement, AbstractBandedMatrix}...) = vcat(BandedMatrix.(x)...)
	Base.vcat(x::Union{OneElement, AbstractBandedMatrix}...) = vcat(convert.(BandedMatrix, x)...)