Add configurable handling for RTC minimum value thresholding (WIP)

[gshiroma]

This PR adds configurable options for handling RTC minimum value thresholding during RTC correction, as an alternative to PR #48. The new modes provide greater control over how samples with low RTC area factors are treated. Very low RTC area factors can lead to extremely large gains, approaching infinity when the RTC area factor is zero. This case is currently handled by marking the corresponding backscatter values as invalid (NaN).

This PR introduces five RTC minimum value handling modes, configurable through the GCOV runconfig:

• DISABLED: No minimum-value thresholding is applied.
• CLIP: RTC values below the minimum threshold are clipped to the minimum value.
• INVALID: Samples with RTC values below the threshold are marked as invalid.
• BYPASS_RTC: RTC correction is bypassed for samples below the threshold.
• TRANSITION: RTC correction is bypassed with a smooth transition between rtc_transition_value_db and rtc_min_value_db.

In addition, this PR adds support for a configurable transition start value (rtc_transition_value_db) when using the TRANSITION mode.