Save latest spectra

[erwanp]

Save N latest computed spectra to improve performances when computing the same one twice

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The SpectrumFactory (machinery behind calc_spectrum) can be connected to a SpecDatabase, which is really only a local folder, to automatically retrieve a Spectrum if it's already there; and store it if it's not.

This is not implemented at the calc_spectrum level, but it could be done quite easily (already asked for a few years ago in radis/radis#135 ). The api would be something like :

s = calc_spectrum(..., local_folder="~/radis_spectra")  # automatically retrieve spectra if needed, computes and stores then if not  

Advantages :

• similarities & tests are kept at the Radis backend level; we're sure Radis-App is not mistakently returning a different spectrum it thoughts was the same
• Radis has HDF5 read/write of spectra, so ready/writing will be almost instantaneous (if we're smart in the computation conditions ; retrieving a spectrum on a narrower spectrum than the one already computed would work too )
• no need to change much in Radis-app
• we'd probably only have to add a feature to keep only the N latest spectra ; or clean the cache periodically.

Originally posted by @erwanp in #619 (comment)