Make fields hash:able

[jagot]

At the moment, electric fields are not hash-stable, which limits cacheability of calculations:

julia> @field(F) do
           I₀ = 100u"TW/cm^2"
           λ = 800u"nm"
           τ = 200.0
           σoff = 4.0
           σmax = 6.0
           env = :trunc_gauss
       end;

julia> hash(F)
0xe98a24d6626e286f

julia> @field(F) do
           I₀ = 100u"TW/cm^2"
           λ = 800u"nm"
           τ = 200.0
           σoff = 4.0
           σmax = 6.0
           env = :trunc_gauss
       end;

julia> hash(F)
0x4fbe5f826fee83a1