Revive D3D get_n1_bradial_parameters

[yumouwei]

Implemented changes

• Reimplement D3DPhysicsMethods.get_n1_bradial_parameters which was previously removed in release v0.8 (Release v0.8 #263)
• Create D3DPhysicsMethods.get_btor and use it in get_n1_bradial_parameters and get_n1rms.
• These new physics methods return n_equal_1_mode, n_equal_1_normalized (tested & XFAIL), and btor (not tested)

Notes on testing

• n_equal_1_mode and n_equal_1_normalized are both marked as XFAIL as all 4 testing shots fall within the range of shots where the raw signal data need to be retrieved from files previously stored on iris.
• I have checked these two signals with the last shot (177061) in the DISRUPTION_WARNING table and the outputs from this physics method match the data in the table.
• The outputs for shot 175552 (referenced in Nucl. Fusion 59 (2019) 096016) do not match the stored data in the DISRUPTION_WARNING table. I found out that these stored data match those from the legacy DUD system (fallback data source in the method's logic) instead of the ONFR system which is available for this shot.

Future TODO

• Implement get data from storage for shots between 156199--172430 and 176030--176912 (described in DIII-D unaccessible PTDATA pointnames  #261).