Prevoiusly, disabling an oximeter session could cause bookmarks to be
disabled and/or OSCAR to crash. OSCAR was looking up the session for
the oximeter session and not noticing it got a Journal session instead.
- make Day::validPressure() available
- Welcome page to report n/a if pressure not valid (but it should always be valid)
- Did NOT add validPressure() call to all pressure reports.