Different datatypes in fits files

[dsavchenko]

This is to extend fits format support. Fits may contain different data types (image/spectra/lightcurve etc) and it's useful to differentiate between them esg-epfl-apc/tools-astro#115

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• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
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  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

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