Crash when using ZFP compression with fixed-precision mode

Created by: keichi

If precision parameter is specified, itMode set in itMode = operation.Parameters.find("precision"); gets overwritten by the following itMode = operation.Parameters.find("rate"); and const std::string modeStr = itMode->second; will cause a crash.

https://github.com/ornladios/ADIOS2/blob/c54e0b11aaef639fd30d09d5d97ce5341f285a68/source/adios2/toolkit/format/bp3/operation/BP3Zfp.tcc#L61-L81