Check the size of the output data proportional to the size of the input data

[andrewphilipsmith]

As a:
user who is processing large volumes of data
I want:
alto2txt to check the ratio of the size of the output data to the size of the input data and warn me if it falls outside a given range.
so that:
potential errors in the text processing can be identified.

Notes:

• The ratio of input/output should be calculated on the smallest unit practical; eg "newpaper+year", or "newpaper+month" or "newpaper+issue" etc.
• The acceptable range of ratios will need to be worked out experimentally (though anecdotally around 10:1 seems typical).
• There should be unittests to accompany this.

This idea came out of a meeting with @fedenanni and @thobson88. Our notes are here https://hackmd.io/KeOzeaMYTOiF37pGUjUl7A