BOV reader doesn't understand relative path names

[clyne]

The BOV importer doesn't understand relative file path names specified by the DATA_FILE keyword in a .bov file. If the value of DATA_FILE is not an absolute path name it should be interpreted relative to the path to the .bov file that contains the DATA_FILE keyword.

To reproduce:

1. Copy data/Source/BOV/256f.{bin,bov} to another location
2. Edit 256f.bov, changing the value of DATA_FILE to a relative path, e.g.:
   DATA_FILE: ./256f.bin

or simply

DATA_FILE: 256.bin

3. Start vapor, import the .bov file and try to render the data.