Hard-coded constants and updated commenting in DIN99*

Minor fixes to the conversion functions. I'm not positive if this will improve performance significantly or not with the Julia JIT, but it's generally good practice to hard-code constants anyhow. Also provided better commenting as to why `kch` and `ke` are set to 1; the standard provides for them, but does not recommend their use. If in the future someone really needs them, they are available.