Dtype in Info object (and other useful attributes to have?)

[theXYZT]

Based on #472, I noticed that the General Info class could expose a few more attributes that are useful. Here are a couple I was surprised to see weren't provided:

1. dtype: Weird this isn't explicitly available as complex_data is provided and, from reading the code in base/base.py, it appears that dtype is inferred via np.complex64 if self.complex_data else np.float32

2. sample_shape: Since shape is provided, shouldn't sample_shape be provided as well?

[mhvk]

Going over old issues: yes, this makes sense. I thought initially that info should just provide a minimal set, but really dtype is useful. And easy to do, just add some parent attributes to base.file_info.StreamReaderInfo...

p.s. I'd also like to add some more control over the exact dtype. Perhaps float16 for some cases, or even allow integer representation to be used. For incoherent dedispersion plus gating and then writing out, one does not need the transformation to float.