Work in progress: ColumnFromBroadcast and ColumnFromAggregation

[smmaurer]

This PR adds two more derived variable templates. This continues the implementation of issue #98. Like the first column template, they use settings objects (PR #108).

ColumnFromBroadcast

Template to register a column of derived data, generated by mapping values from another table via one or more join keys. Values calculated lazily.

Parameters:

meta : CoreTemplateSettings
data : BroadcastSettings
- source_table  # either single table or chain
- expression  # either single column or expression
output : OutputColumnSettings

Example use cases:

• add tract population in thousands to the households table

ColumnFromAggregation

Template to register a column of derived data, generated by aggregating values from another table using group-by with a join key. Values calculated lazily.

Parameters:

meta : CoreTemplateSettings
data : AggregationSettings
- source_table  # either single table or chain
- expression  # either single column or expression (evaluated before grouping)
- filters  # filter rows before grouping
- group_by
- aggregation  # min, max, mean, count, sum, stdev, etc.
output : OutputColumnSettings

Example use cases:

• calculate median building size by census tract
• calculate total single-family housing units in a zone, in thousands

Versioning

• 0.2.dev7, probably

To do before merging

• revise ColumnFromBroadcast to use settings objects
• finish testing ColumnFromBroadcast
• implement and test ColumnFromAggregation
• finalize versioning
• update docs and changelog