Fix unsafe loading from file buffer

[felixthebeard]

The numpy documentation states for numpy.frombuffer` that:

This function creates a view into the original object. This should be safe in general, but it may make sense to copy the result when the original object is mutable or untrusted.

When the filebuffer is mutable a ValueError: output array is read-only is raised in: https://github.com/eyurtsev/fcsparser/blob/701000af178e36e0dedb53d409119c25675b9d16/fcsparser/api.py#L590

[eyurtsev]

Thanks @felixthebeard