Fix replace! with different `BC` types in nodes, faces and elements boundary conditions

[Mauricio Vanzulli]

julia> typeof(boundary_conditions(s).node_bcs)
Dictionaries.Dictionary{FixedDof, Vector{Node{1, Float64}}}

julia> haskey(boundary_conditions(s).node_bcs, constant_gravity)
ERROR: MethodError: Cannot `convert` an object of type GlobalLoad to an object of type FixedDof

Closest candidates are:
  convert(::Type{T}, ::T) where T
   @ Base Base.jl:64

Stacktrace:
 [1] in(i::GlobalLoad, indices::Dictionaries.Indices{FixedDof})
   @ Dictionaries ~/.julia/packages/Dictionaries/7aBxp/src/AbstractIndices.jl:73
 [2] haskey(dict::Dictionaries.Dictionary{FixedDof, Vector{Node{1, Float64}}}, i::GlobalLoad)
   @ Dictionaries ~/.julia/packages/Dictionaries/7aBxp/src/AbstractDictionary.jl:37
 [3] top-level scope
   @ REPL[12]:1