* for -O3 gcc generated wrong code for these loops

* this issue could result in decomposition dependent interpolation results

initialised double precision buffer in yac_fget for the r4 case in order to avoid issues with NaN when returning this buffer

The correspondings functions return iso8601 strings instead.
The user decides what to do and how to convert these strings.

* we install with pip and do not need to check for the installation
  subdirectories

* this makes the option and the variable more visible in the help message