`tod.concatenate` doesn't support group with mandatory `__init__` keyword arguments

[ljgray]

This is a bit tricky, but probably something that should be supported.

If you make a subclass of MemDiskGroup (or BasicCont) with additional required arguments in the constructor, trying to concatenate several instances of this class will fail because tod.concatenate needs to be able to initialize a bare container.

I'm not entirely sure if/how this can be addressed, but it's worth making note of the issue.