Adding merge_sample() method

[brimoor]

Resolves #3273

Adds a merge_sample() method for merging a single sample into a dataset via the same basic interface as merge_samples().

This method is useful for workflows that process one sample at a time, eg a cloud function that processes one sample at a time. It is not intended for use in a loop:

# Don't do this
for sample in samples:
   dataset.merge_sample(sample, ...)

# Use this instead
dataset.merge_samples(samples, ...)