Add data input check to ensure expression counts are integers

[catavallejos]

Currently there is no warning if the data has been pre-normalised (e.g. using scran)

[alanocallaghan]

Not all aligners/counters produce integers, eg https://bmcbioinformatics.biomedcentral.com/articles/10.1186/1471-2105-12-323

I've come across this before, and the NB is defined for non-integer values, so there might be a smarter way to figure out if the data have been normalised (something like https://www.nxn.se/valent/2018/10/25/unscaling-scaled-counts-in-scrna-seq-data )