Option for `+` behavior

[jpivarski]

• "fast": ignore binning metadata and just add counts arrays naively
• "adjust": the current behavior and future default; moves bin positions or creates bins (with zero content) to make binnings compatible, then add
• "strict": raise an exception (ValueError) if binnings disagree (in a way that would make "fast" invalid).

(At the suggestion of @benkrikler.)