A pseudo string stream to allow for more low-level efficient implementations of formatting functions speficic to elPrep.
The struct sim-stream has a constructor make-sim-stream that takes an optional initial size for the target string as a parameter.
Accessor sim-stream-string refers to the target string for output.
Accessor sim-stream-index refers to the current index into sim-stream-string.
Accessor sim-stream-%floats refers to an optional actual string-stream for floating point output.