Force SymTridiagonal to have consistent lengths

[musoke]

#629 pointed out that is possible to initialise SymTridiagonal with subdiagonal of length equal to the diagonal. This is inconsistent with Tridiagonal and Bidiagonal.

Check the lengths and error if the subdiagonal's length is not 1 less than the diagonal's length.

Remove tests for SymTridiagonal with length(ev) == length(dv).

Todo:

• add test that len(dv) == len(ev) now errors
• remove tests with len(dv)==len(ev)
• fix kron
• @andreasnoack mentioned that changes may be need in the wrapper for dstemr.
  I don't think that any changes are needed for dstemr. As far as I can tell from LAPACK exception thrown from eigen(::SymTridiagonal) #1491, dstemr is called via dstgr!. It already has checks in place for the lengths of de and ev:
  

  LinearAlgebra.jl/src/lapack.jl

  Lines 4043 to 4058 in 17325b7

  	function stegr!(jobz::AbstractChar, range::AbstractChar, dv::AbstractVector{$elty}, ev::AbstractVector{$elty}, vl::Real, vu::Real, il::Integer, iu::Integer)
  	require_one_based_indexing(dv, ev)
  	@chkvalidparam 1 jobz ('N', 'V')
  	@chkvalidparam 2 range ('A', 'V', 'I')
  	chkstride1(dv, ev)
  	n = length(dv)
  	ne = length(ev)
  	if ne == n - 1
  	eev = [ev; zero($elty)]
  	elseif ne == n
  	eev = copy(ev)
  	eev[n] = zero($elty)
  	else
  	throw(DimensionMismatch(lazy"ev has length $ne but needs one less than or equal to dv's length, $n)"))
  	end

Possible followups:

• simplify _evview, now that ev always has the same length.
• remove most calls to _evview

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.30%. Comparing base (17325b7) to head (8510362).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1569      +/-   ##
==========================================
- Coverage   94.33%   94.30%   -0.03%     
==========================================
  Files          35       35              
  Lines       16007    16009       +2     
==========================================
- Hits        15100    15098       -2     
- Misses        907      911       +4     

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[musoke]

I think this is ready for review. @andreasnoack, could you take a look?