Create an inspection function to determine stream properties

[mhvk]

It would be useful to have a function that can inspect a stream, tell which type it is, what its basic properties are, and whether it seems basically correct (e.g., do tests on EDV3 that the sample rate makes sense).

Obviously, this would be a requirement for eventually having a baseband.open()

[mhvk]

This information should include sample rate from VDIF headers (and whether it is correct or not; see #135)

Also see the VDIF frameset consistency checker #117

[mhvk]

First cut at this in #198 (not for 1.0).

[mhvk]

fixed by #200