Make RaySolver work with Measurement data type

[mchitre]

Measurement from Measurements.jl fails with RaySolver.

Example:

julia> env = UnderwaterEnvironment();
julia> pm = RaySolver(env);
julia> f(d) = transmissionloss(pm, AcousticSource(0.0, -10.0, 1000.0), AcousticReceiver(d, -7.0))
f (generic function with 1 method)

julia> f(1000.0)
52.65450803568578

julia> f(1000.0 ± 1.0)
ERROR: TaskFailedException
    nested task error: MethodError: no method matching Float64(::Measurement{Float64})
      ...
    
    Stacktrace:
      [1] convert(#unused#::Type{Float64}, x::Measurement{Float64})
        @ Base ./number.jl:7
      ...
      [9] perform_step!(...)
      ...
     [11] solve!(...)
      ...
     [19] traceray(...)
        @ AcousticRayTracers ~/Projects/AcousticRayTracers.jl/src/RaySolver.jl:248
     [20] traceray (repeats 2 times)
        @ ~/Projects/AcousticRayTracers.jl/src/RaySolver.jl:231 [inlined]
     [21] macro expansion
        @ ~/Projects/AcousticRayTracers.jl/src/RaySolver.jl:71 [inlined]
      ...