Manpage clean-up

[jepasan]

I've gone through the reference pages and cleaned up some unwieldy example outputs as suggested in #450. I mainly spotted three big issues.

• In 41688b42620ef319909e51bb08dff6ddf13b3f49, I've used head() to avoid printing the whole probability matrix in https://microbiome.github.io/mia/reference/calculateDMN.html.
• In 13a55fb98987868c67f8522195f4e81a4f6f54f9, I've removed rows with zero standard deviation from the assay to eliminate the large warning chunks in https://microbiome.github.io/mia/reference/getExperimentCrossAssociation.html. The second assay used in the example also has some NA values, but I haven't changed those since they don't produce too many warnings, and one of the examples demonstrates the use of show_warnings=FALSE.
• In ecf30870a57b1715ecc8f9323e8d362efe22f587, I've reduced the number of artificially created groups to reduce repetitive warnings in https://microbiome.github.io/mia/reference/splitOn.html. These warnings ultimately stem from addTaxonomyTree, which is called for each split object created from the original when update_rowTree = TRUE. I don't think this is the best solution, perhaps these could be suppressed instead?

In addition, I've fixed some grammar mistakes here and there.

[antagomir]

Thanks!

Could @TuomasBorman check what to do with "n https://github.com/microbiome/mia/commit/ecf30870a57b1715ecc8f9323e8d362efe22f587, I've reduced the number of artificially created groups to reduce repetitive warnings in https://microbiome.github.io/mia/reference/splitOn.html. These warnings ultimately stem from addTaxonomyTree, which is called for each split object created from the original when update_rowTree = TRUE. I don't think this is the best solution, perhaps these could be suppressed instead?"