Fix GPU b-field initialization

[shahor02]

The 12kA L3 field should be scaled wrt its own nominal rather than 30kA nominal Bz. Keep the old scaling for backward compatibility.

Don't merge until TPC CTF decoding with field Bz from the CTF itself is not ready, as this PR will crash low-field data decoding.

Example of difference:

// original field for -12kA:
auto f2 = o2::field::MagneticField::createFieldMap(-12000, -6000);
f2->GetBz(120, 120, -20)
(double) -2.0522978

// old gpu field initialized with -12kA:
fld.GetFieldBz(120, 120, -20)/0.000299792458f
(float) -2.00946f
// -> 2.1% difference

// field with this PR initialized with -12kA 
fld.GetFieldBz(120, 120, -20)/0.000299792458f
(float) -2.05180f
// -> 0.024% difference

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