data structure for affine expressions

[lucaferranti]

PR description

Preliminary structure for linear expressions for PILS.

Quick showcase

julia> @linvars x y z
z

julia> p1 = x + y + z + 1
x+y+z+1

julia> p1([1, 1, 1])
4

julia> p1 - 2
x+y+z-1

julia> p2 = x + y
x+y

julia> p1 - p2
z+1

Related issues

Checklist

• [x] Updated/added tests
• [x] Updated/added docstring (I need to add docstrings for AffineExpression and @linvars)

Other

[codecov-commenter]

Codecov Report

Merging #104 (92109df) into main (dd1de39) will increase coverage by 6.21%. The diff coverage is 98.75%.

@@            Coverage Diff             @@
##             main     #104      +/-   ##
==========================================
+ Coverage   90.96%   97.18%   +6.21%     
==========================================
  Files          14       15       +1     
  Lines         487      568      +81     
==========================================
+ Hits          443      552     +109     
+ Misses         44       16      -28     

Impacted Files	Coverage Δ
src/pils/linexpr.jl	98.73% <98.73%> (ø)
src/IntervalLinearAlgebra.jl	100.00% <100.00%> (ø)
src/linear_systems/enclosures.jl	85.52% <0.00%> (+0.19%)	:arrow_up:
src/linear_systems/oettli.jl	100.00% <0.00%> (+50.00%)	:arrow_up:
src/linear_systems/oettli_nonlinear.jl	100.00% <0.00%> (+100.00%)	:arrow_up:

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[lucaferranti]

I'm merging this since it's needed to get the first prototype of PILS solver (see #113 ). The idea is that this is a small simple symbolic functionality to make the package "battery included", but of course users could (and should) use e.g. Symbolics.jl or other stronger packages if they need symbolic manipulation. The AffineParametricArray is parametrised on the type of variables, so it should work (once the code to parse input from Symbolics.jl is added, which is delegated to an upcoming PR)