Adding timezone handling functionality

[vincentvanhees]

This PR adds timezone handling, see issue #43.

1. Two new input arguments:

• configtz => the timezone where the accelerometer was configured.
• desiredtz => the timezone where the accelerometer was worn.

I am using the names configtz and desiredtz because that is the terminology I have been using in R package GGIR. However, we are of course not bound by those names. So, if you prefer alternative more intuitive names then we can do that. For example: tz_worn and tz_config.

2. To preserve consistency with previous read.gt3x releases, the default behaviour with configtz=NULL and desiredtz=NULL leaves default tz GMT untouched.
3. Added unit tests for timestamp covering both old- and new .gt3x format.
4. Gives warning when only one of the two timezone arguments is specified, and then assumes that the unspecified tz equals the specified tz.
5. Gives error if timezone name is not in OlsenNames().
6. Added unit tests for this warning (4) and error (5).