Update SMAP L1C DA to use dynamic Tb_error information to inflate observation errstd

[gmao-qliu]

This PR updates read_obs_SMAP_halforbit_Tb to utilize the dynamic tb_error values provided in the SMAP L1C data to change the observation error standard deviation (errstd). The purpose is to reduce the impact of the RFI-affected Tbs in the SMAP L4 system.

Previously, errstd for SMAP L1C Tb was set at a constant 4K. By reading and applying the actual L1C Tb_error, the assimilation system now increases the error of Tb observations based on their provided Tb_error values using:

errstd = errstd * inflation_factor
where:
inflation_factor = exp(20 * (Tb_err - 1.3))
and
inflation_factor is limited to [1.0, 20.0]

Impact & Expected Behavior
This change only impacts the SMAP L1C Data Assimilation. No other sensors or general DA settings were touched.
Results will remain identical to the previous implementation when Tb_error <= 1.3K.
Assimilation results will diverge from the baseline in instances where Tb_error > 1.3K.

Testing Done
Verified that the code compiles and runs without structural errors.
Confirmed Tb_errors are predominantly < 1.3K in normal circumstances.
Confirmed expected numerical deviations in assimilation for cases where Tb_error > 1.3K (e.g., due to low-level RFI).