Fix printing of SparseOpPureType

[apkille]

See #45.

julia> create(FockBasis(2)) # before
Operator(dim=3x3)
  basis: Fock(cutoff=2)sparse([2, 3], [1, 2], ComplexF64[1.0 + 0.0im, 1.4142135623730951 + 0.0im], 3, 3)

julia> create(FockBasis(2)) # after
Operator(dim=3x3)
  basis: Fock(cutoff=2)
      ⋅                           ⋅            ⋅     
 1.0 + 0.0im                      ⋅            ⋅     
      ⋅       1.4142135623730951 + 0.0im       ⋅    

julia> QuantumOpticsBase.set_printing(standard_order = true)

julia> create(FockBasis(2))
Operator(dim=3x3)
  basis: Fock(cutoff=2)
  [2, 1]  =  1.0+0.0im
  [3, 2]  =  1.41421+0.0im

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.04%. Comparing base (bb71f52) to head (cc67ebd).
Report is 21 commits behind head on master.

Files with missing lines	Patch %	Lines
src/printing.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #195      +/-   ##
==========================================
+ Coverage   93.00%   93.04%   +0.04%     
==========================================
  Files          25       26       +1     
  Lines        3104     3266     +162     
==========================================
+ Hits         2887     3039     +152     
- Misses        217      227      +10     

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[Krastanov]

Thank you!!! This has been an eyesore for a long time.