Add HessianTracer

[adrhill]

Closes #20

CC: @gdalle @Vaibhavdixit02

Example

julia> using SparseConnectivityTracer

julia> x = rand(5);

julia> f(x) = x[1] + x[2]*x[3] + 1/x[4]
f (generic function with 1 method)

julia> H = pattern(f, HessianTracer, x)
5×5 SparseArrays.SparseMatrixCSC{Bool, UInt64} with 3 stored entries:
 ⋅  ⋅  ⋅  ⋅  ⋅
 ⋅  ⋅  1  ⋅  ⋅
 ⋅  1  ⋅  ⋅  ⋅
 ⋅  ⋅  ⋅  1  ⋅
 ⋅  ⋅  ⋅  ⋅  ⋅

julia> g(x) = f(x) + x[2]^x[5]
g (generic function with 1 method)

julia> H = pattern(g, HessianTracer, x)
5×5 SparseArrays.SparseMatrixCSC{Bool, UInt64} with 7 stored entries:
 ⋅  ⋅  ⋅  ⋅  ⋅
 ⋅  1  1  ⋅  1
 ⋅  1  ⋅  ⋅  ⋅
 ⋅  ⋅  ⋅  1  ⋅
 ⋅  1  ⋅  ⋅  1

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 49.64029% with 70 lines in your changes are missing coverage. Please review.

Project coverage is 54.91%. Comparing base (475e576) to head (cc9f808).

Files	Patch %	Lines
src/overload_hessian.jl	17.74%	51 Missing ⚠️
src/tracers.jl	69.23%	16 Missing ⚠️
src/conversion.jl	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #24      +/-   ##
==========================================
- Coverage   63.41%   54.91%   -8.50%     
==========================================
  Files           7        8       +1     
  Lines         123      244     +121     
==========================================
+ Hits           78      134      +56     
- Misses         45      110      +65     

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