Cashe timezone so that timezone file only has to be accessed once per simulation.

[jhs507]

Currently simulations run very slowly on certain systems because in StandardConverterUtility.cpp on line 83 and 105 the function mktime is called which looks up the timezone of the local system. This look up takes a significant amount of time on certain systems where this file is stored on a slow drive.

The time zone should be cashed after being retrieved once and a call which does not require a system lookup should be used instead.

[jhs507]

Setting the TZ environment variable will cause that to be used instead of having to lookup the timezone on each use of mktime.

I will find a way to do this when CRHM is initialized.

[jhs507]

This will solve the issue talked about in #379 in a way that doesn't invalidate previous models.