Reindex flags refactoring + match method fix

Tiny rework of reindex flags. Syntax now is as follows:

saqc.concat(field='a', target='b', method='x')

The call is to be read as: Append history of a to history of b, and use method x for resampling the row index.

Also fixed the match method keyword (appending with no resampling method applied. Preassumes field and target to have identical indices.)