Allow `Dt` as alternative keyword in `trajectory`

[codecov-commenter]

Codecov Report

Merging #173 (e675f5b) into main (6fae44b) will decrease coverage by 0.89%. The diff coverage is 55.55%.

@@            Coverage Diff             @@
##             main     #173      +/-   ##
==========================================
- Coverage   82.88%   81.99%   -0.89%     
==========================================
  Files          14       15       +1     
  Lines         701      711      +10     
==========================================
+ Hits          581      583       +2     
- Misses        120      128       +8     

Impacted Files	Coverage Δ
src/core_systems/continuous_time_ode.jl	86.79% <ø> (+1.60%)	:arrow_up:
src/derived_systems/poincare/poincaremap.jl	79.00% <ø> (ø)
src/core/trajectory.jl	82.00% <33.33%> (-15.50%)	:arrow_down:
src/derived_systems/projected_system.jl	93.02% <100.00%> (ø)

... and 2 files with indirect coverage changes

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