Stop after a number of readings and "one-shot" mode

[avaldebe]

From #1

2. Allow readings on demand, returning readings in an appropriate structure, so that the timing and storing of data can be managed by the python app using PyPMS?

A more general use case would be to stop the data acquisition after a number successful observations. Two new options will be introduced:

• -n N, --samples N: stop the data acquisition after N successful observations.
• --one-shot equivalent to -n1 --strict, where --strict takes care of invalid observations after wake up (as discussed on #1 and #3).

[avaldebe]

As there is no need for a --strict flag (see #3), there is no need for a --one-shot flag either.