Change argument checking in SoundFile.__init__()

[mgeier]

This is an alternative to #60. Both are possible solutions to #59.

Advantages of this solution:

• the built-in open() function is not needed
• the file argument is straightforwardly passed to libsndfile

Disadvantages of this solution:

• the error messages after failing to open a file are just guesses. They may be misleading?
• if arguments are specified even if they are not needed, they are ignored (instead of raising an error)

More or less neutral points:

• the behavior is closer to libsndfile than to Python's open() function
• read and write positions can be handled independently

[mgeier]

I'm closing this because we decided to use #60 instead.