Make Physics-specific BCType

[JaredCrean2]

Currently BCType is defined in ODLCommonTools and is the supertype of all boundary condition functors, even those in different physics modules. It might be beneficial to have something like

abstract type BCType end

# in physics module Euler
abstract EulerBCType <: BCType
mutable struct FreeStreamBC <: EulerBCType
end

# in physics module NavierStokes
abstract NSBCType <: BCType
mutable struct noSlipBC <: NSBCType
end

This would prevent the Euler module from accidentally trying to evaluate a NavierStokes boundary condition.