Fix issues with last_updated in MapBuilder

[shyamd]

This PR fixes some issues with the treatment of last_updated in the core store and MapBuilder

• make newer_in more resilient to lack of last_updated fields
• fix typos in MultiplyBuilder example

[codecov]

Codecov Report

Merging #98 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #98   +/-   ##
=======================================
  Coverage   87.19%   87.19%           
=======================================
  Files          20       20           
  Lines        1453     1453           
=======================================
  Hits         1267     1267           
  Misses        186      186

Impacted Files	Coverage Δ
src/maggma/core/store.py	87.23% <ø> (ø)	⬆️
src/maggma/builders/map_builder.py	91.2% <ø> (ø)	⬆️

---

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 ae1c09f...e985114. Read the comment docs.