BMD boundary check for BMD censoring arguments & update documentation

[sedavid01]

The current arguments for bmd censoring are unrestricted in their values. Thus, users can input values that actually do not make sense for the purpose of those arguments.

Goal: Add checks within tcplhit2_core function to ensure the multipliers for the bmd censoring arguments are within their allowable ranges. Also, update the documentation to better clarify what the boundaries are meant to do.

• bmd lower bound multiplier (b_l) -> 0 < b_l <= 1 (basically between zero and 1 not including 0)
• bmd upper bound multiplier (b_u) -> b_l >= 1 (basically any value greater than 1)

Note: We may want to put a warning for values that are super extreme. So b_l <= 1e-3 and b_u >=1000. (@kpaulfriedman may have an alternative suggestion for these extreme warning cases)

[sedavid01]

Suggest warning message is something to the effect of:

• lower threshold warning message: "Warning: The specified bmd_lower_bnd is less than 1e-3, and may result in an extremely low threshold value for BMD censoring. Typically recommended value is 0.1."
• upper threshold warning message - something similar as above for this case.

[sedavid01]

Pull request was approved and merged to 'dev'. Closing this ticket out.