Show warning in gsBoundSummary() when hr0 is not present

[nanxstats]

gsDesign::gsBoundSummary() should be updated to show at least a warning when hr0 is not present and before it auto-switches to the default value 1. This will make the design update less error-prone.

Assume x is the original design and xu is the updated design, the current workaround is

For non-inferiority and super-superiorty trials
xu$hr0 <- x$hr