Avoid a deepcopy when constructing MPBP from SIS

[stecrotti]

mpbp(sis::SIS) and similar become quite slow for large graph sizes / long times. Profiling shows most of the time is spent at this call to deepcopy. This PR eliminates it

Warning: let's check first that this doesn't break anything

Benchmark

[crotti@sibyl MatrixProductBP]$ git checkout main
Switched to branch 'main'
Your branch is up to date with 'origin/main'.
[crotti@sibyl MatrixProductBP]$ julia -e '
> using MatrixProductBP, MatrixProductBP.Models, Graphs, IndexedGraphs
> T = 10^3        # final time
> k = 3         # degree
> λ = 0.07      # rate of transmission
> ρ = 0.1       # rate of recovery
> γ = 0.2      # prob. of zero patient
> N = 5*10^3
> gg = random_regular_graph(N, k; seed=0)
> g = IndexedGraph(gg)
> sis = SIS(g, λ, ρ, T; γ)
> # run once for compilation
> bp = mpbp(sis)
> # measure runtime
> @time mpbp(sis)
> '
225.252552 seconds (160.36 M allocations: 9.850 GiB, 26.75% gc time)
[crotti@sibyl MatrixProductBP]$ 
[crotti@sibyl MatrixProductBP]$ git checkout no-copy
Switched to branch 'no-copy'
Your branch is up to date with 'origin/no-copy'.
[crotti@sibyl MatrixProductBP]$ 
[crotti@sibyl MatrixProductBP]$ julia -e '
> using MatrixProductBP, MatrixProductBP.Models, Graphs, IndexedGraphs
> T = 10^3        # final time
> k = 3         # degree
> λ = 0.07      # rate of transmission
> ρ = 0.1       # rate of recovery
> γ = 0.2      # prob. of zero patient
> N = 5*10^3
> gg = random_regular_graph(N, k; seed=0)
> g = IndexedGraph(gg)
> sis = SIS(g, λ, ρ, T; γ)
> # run once for compilation
> bp = mpbp(sis)
> # measure runtime
> @time mpbp(sis)
> '

 40.912102 seconds (140.32 M allocations: 6.984 GiB, 58.05% gc time)

shows more than x5 speed-up

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 90.69%. Comparing base (4570681) to head (102be4f).

Files	Patch %	Lines
src/Models/epidemics/sis_heterogeneous_bp.jl	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #128      +/-   ##
==========================================
+ Coverage   90.65%   90.69%   +0.04%     
==========================================
  Files          22       22              
  Lines        1305     1300       -5     
==========================================
- Hits         1183     1179       -4     
+ Misses        122      121       -1     

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