Add date units to periods

[bonjourmauko]

Depends on #1138

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New features

• Introduce DateUnit to periods
  • Allows for transparent encapsulation of the atomic units of periods (year, month, ...).
  • This helps with testing, refactoring and exrtension of periods (weeks for example).

[coveralls]

Coverage increased (+0.7%) to 79.676% when pulling f51fc6252ba9ff6f101cdc09cf06f84a619da86b on add-date-units into 3eb157f20ebcad6a87ecc583c00f266dc22dca0e on master.

[benjello]

Looks nice @maukoquiroga.

[bonjourmauko]

Superseded by #1138