Allow relative sample size input in subsampling

[nkongenelly]

This code allows subsampling to work with relative numbers too as the initial implementation limited it to absolute number of reads (#50).
seqtk_sample module itself allows passing fractions as the sample_size thus only our schema has been updated here to allow both integers and floats

Fixes #60

Co-authored-by: agrima2010
Co-authored-by: kjelinjonas

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