boot_strata changes

[gravesti]

• fix check in estimate weights to first check n bootstrap samples > 0 or non-null, then check bootstrata column exists
• set defatul boot_strata = NULL
• make example showing how to use boot_strata

[MikeJSeo]

can we also uppercase ARM, USUBJID in estimate_weights function? would this fix the issue?

Another thing:
result_boot$inferential$boot_est does not exist in anchored_maic for survival. It exists in binary.

[kurttaylor]

boot_strata = NULL should definitely be default to prevent older code from erroring!