Remove hardcoded intervals

[dajmcdon]

• level is a mandatory arg to the method. This should be used to select the confidence level.
• It may also be nice to allow multiple levels rather than one (but this complicates plotting).

[kaitejohnson]

Was thinking about this-- I think we want to make sure that all of the sumrmt_summary objects are by default generating the same CI band level (I had assumed they were all 95% CI but now am not sure this was ever verified). I think it makes sense for this to be the default but allow the user to specify.