Bugfix - using scripts from a different directory

I'm using ostn02python as part of a larger project, and I've found that scripts fail when the ostn02python.txt.bz2 file isn't in the current directory (example below). This patch fixes that.

```
rob@rivertam:~$ python ostn02python/example.py 
Traceback (most recent call last):
  File "ostn02python/example.py", line 12, in <module>
    from OSTN02 import *
  File "/home/rob/ostn02python/OSTN02.py", line 32, in <module>
    ostn_data = ostn() # load all the data from below
  File "/home/rob/ostn02python/OSTN02.py", line 21, in ostn
    fi = bz2.BZ2File("ostn02data.txt.bz2")
IOError: [Errno 2] No such file or directory: 'ostn02data.txt.bz2'
```