sunpos_la(), gettz(): make it easier to use these routines stand alone

* sunpos_la(), gettz(): make it easier to use these routines stand alone:
  * some input variables could only be passed using modules
  * now, optional arguments were added to so this in a more straightforward way
    * sunpos_la(): latitude, longitude and height
    * gettz(): tz0 and dsttp
  * note that using the optional arguments updates the module variables, but since the
    former is presumably used when the latter isn't, this shouldn't cause many problems