Fix functions overwriting in `heat_equation` tutorial

[ghislainb]

See the log of the CI:

WARNING: Method definition a(Any, Any) in module heat_equation_API at /home/runner/work/BcubeTutorials.jl/BcubeTutorials.jl/src/tutorial/heat_equation.jl:53 overwritten at /home/runner/work/BcubeTutorials.jl/BcubeTutorials.jl/src/tutorial/heat_equation.jl:100.
WARNING: Method definition l(Any) in module heat_equation_API at /home/runner/work/BcubeTutorials.jl/BcubeTutorials.jl/src/tutorial/heat_equation.jl:54 overwritten at /home/runner/work/BcubeTutorials.jl/BcubeTutorials.jl/src/tutorial/heat_equation.jl:102.

Possible solutions:

• split the tutorial in two files or in two functions to define the functions in different scope
• use different function names