Cleanup when writing floats as strings not working as expected

[asmaloney]

The impact is negligible since it's just the floating point representation in the XML portion of the file.

However, the code & comments indicate that these floating point numbers should be cleaned up when output, so 1.23456000000000000e+005 would become 1.23456e+005 and 2.00000000000000000e+005 would become 2e+005.

If we look at the new tests (see test_StringFunctions.cpp), it shows that the exponent parts may only be two digits instead of three (e.g. e+05) and so none of the cleanup happens (we end up with e+00).

I'm also not sure why all this effort is put in here to then leave e+005 instead of making it e+5...