Add missing istree method for QTerm types

[david-pl]

We were missing the istree(::Type{<:QTerm})=true method, so similarterm returned the wrong thing. This lead to errors in substitute (see also #93).

[codecov]

Codecov Report

Merging #96 (5b93aaa) into master (aa5cfa9) will decrease coverage by 0.16%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #96      +/-   ##
==========================================
- Coverage   83.40%   83.24%   -0.17%     
==========================================
  Files          16       16              
  Lines        1742     1743       +1     
==========================================
- Hits         1453     1451       -2     
- Misses        289      292       +3     

Impacted Files	Coverage Δ
src/qnumber.jl	90.23% <100.00%> (+0.98%)	⬆️
src/correlation.jl	85.71% <0.00%> (-1.46%)	⬇️

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