working with empty Spectrum object

[wangbingjie]

First attempt to allow for an empty object in obs. I am just creating this PR in case it would be helpful when we are ready to solve this problem properly; i.e., we don't necessarily need to merge this (especially given that I am not entirely sure I know what I am doing with p-v2...). Note that this version does not solve all the errors; specifically, this error remains when writing to an h5 file in observation.py, in to_struct:

if (len(dat) != self.ndata):
TypeError: len() of unsized object

But I am less familiar with the io part....