Update key `iter_samples` to `iter_sampling`

[zsusswein]

It was already documented as iter_sampling in the docs, but the code expected iter_samples.

iter_sampling follows {cmdstanr} syntax: https://mc-stan.org/cmdstanr/reference/model-method-sample.html

iter_samples is an accidental portmanteau of EpiNow2's desired samples arg and iter_sampling.

Also added a few quality-of-life Makefile utilities

Closes #73

[codecov[bot]]

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