Speedstring precompilation warning

[milankl]

fixes #296 by "extending" the speedstring method just with a bit more specific ::AbstractFloat dispatch (in ProgressMeter it's ::Any) not through overwriting

It seems to work

(@v1.9) pkg> add https://github.com/SpeedyWeather/SpeedyWeather.jl#mk/speedstring
    Updating git-repo `https://github.com/SpeedyWeather/SpeedyWeather.jl`
   Resolving package versions...
    Updating `~/.julia/environments/v1.9/Project.toml`
  [9e226e20] + SpeedyWeather v0.5.0 `https://github.com/SpeedyWeather/SpeedyWeather.jl#mk/speedstring`
    Updating `~/.julia/environments/v1.9/Manifest.toml`
  [9e226e20] + SpeedyWeather v0.5.0 `https://github.com/SpeedyWeather/SpeedyWeather.jl#mk/speedstring`
Precompiling project...
  1 dependency successfully precompiled in 17 seconds. 313 already precompiled