Refactor Var types to include unit information

docs/src/manual/overview.md

src/Drivers/DiffEq/DiffEq.jl

src/IO/output.jl

src/Numerics/Numerics.jl

src/Numerics/variables.jl

src/Numerics/varstates.jl

src/Physics/Boundaries/bc.jl

src/Physics/HeatConduction/heat.jl

src/Physics/HeatConduction/heat_bc.jl

src/Physics/Physics.jl

src/Physics/SEB/SEB.jl

src/Physics/SEB/seb_heat.jl

src/Physics/coupled.jl

src/Strat/state.jl

src/Strat/tile.jl

test/Physics/HeatConduction/heat_conduction_tests.jl

test/Strat/tile_tests.jl