some fixes for magnetometer cal

[mikeferguson]

• the spinOnce was needed
• exit properly on CTRL-C

[codecov-io]

Codecov Report

Merging #84 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #84   +/-   ##
=======================================
  Coverage   54.50%   54.50%           
=======================================
  Files          34       34           
  Lines        1730     1730           
=======================================
  Hits          943      943           
  Misses        787      787           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a1d2508...4ee0acc. Read the comment docs.