Removed feature of passing the file as a pathname from filetensor.{read, write} Rationale: 1. The common pattern of multiple successive reads to load a list of arrays doesn't work for the string argument. 2. It isn't clear without checking the docs whether the write() will open a file in append mode, which could lead to bugs. 3. It's a fairly useless feature anyway, because the truly lazy programmer can call write(open(filename, 'w'), mat) to reproduce the old behaviour if he wants it, and without the open-mode ambiguity.

This seems to work.

@todo:
    * Add momentum.
    * Add learning rate decay schedule.