Improvement: don't use absolute path names in metadata cache

[awst-baum]

The ISMN reader seems to create a cache of the ISMN data in a folder in a subfolder called python_metadata in a file called metadata.npy.

metadata.npy contains absolute pathnames of the stm files, e.g. /path/to/mydata/ISMN/ISMN_V20180830_GLOBAL/AMMA-CATCH/Banizoumbou/AMMA-CATCH_AMMA-CATCH_Banizoumbou_sm_0.050000_0.050000_CS616-1_19780101_20180830.stm.

In our setup, we use the same NFS mount containing ISMN data in several systems, and not all systems use the same mountpoint (e.g. one might have the data at /path/to/mydata and the other have it at /some/completely/different/path/to/my/data). Also, we use linux softlinks, so we might also access the data as /softlink/to/data/....

Once the metadata folder is created by one of the systems, the other systems may produce errors if they don't use the same path to access the data because they can't find the stm files at the paths cached in the metadata.npy file.

Would it be possible to not use absolute file paths or make it possible in some other way to use the same ISMN folder from several systems?