New command: prepare

[jpmckinney]

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

• 18

• 15

• 13

• 10

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

• 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.