New command: prepare

[jpmckinney]

Following the principle of "Keep data preparation separate from indicator calculation " #29, we can add a command to do:

• Add configuration option to map to standard codes #18
• Allow values to be absent #15
• Consider support for string "amount" values #13
• Add support for number "id" values #10

and maybe these as possible (opt-in) pre-processing:

• Use parties/identifier instead of organization reference ID #32 change id references to identifier/id (perhaps if consistently available)

Pretty much all issues labeled 'robustness' could be resolved via this command.

Also:

• Lowercase codes. For now, we'll require users to manually map such codes. If it's a common issue, we can add an option that lowercases all codelist fields used in indicators.

[jpmckinney]

The current prepare command is sufficient for DGCP. Other labeled issues remain open. Moved some comments from issue description to those issues.