Change all uses of Kelvin to Celsius

README.md

docs/src/manual/overview.md

docs/src/quickstart.md

examples/heat_freeW_samoylov_simple.jl

examples/heat_vgfc_samoylov_simple.jl

examples/heat_vgfc_seb_samoylov_custom.jl

examples/readme_examples.jl

src/Processes/HeatConduction/HeatConduction.jl

src/Processes/HeatConduction/heat_bc.jl

src/Processes/HeatConduction/soil/sfcc.jl

src/Processes/HeatConduction/soil/soilheat.jl

src/Processes/SEB/seb_heat.jl

test/Processes/HeatConduction/heat_conduction_tests.jl

test/Processes/HeatConduction/sfcc_tests.jl

test/benchmarks.jl