🐛 Avoid eta_cd_dimensionless_hcd_primary = NaN when p_hcd_primary_injected_mw = 0 #4692

Issue

PROCESS calculates three current-drive efficiency quantities for the primary H&CD system:

• eta_cd_hcd_primary: absolute current-drive efficiency in A/W.
• eta_cd_norm_hcd_primary: normalised current-drive efficiency, which scales eta_cd_hcd_primary by plasma density and major radius.
• eta_cd_dimensionless_hcd_primary: dimensionless current-drive efficiency, which additionally normalises the efficiency using the plasma electron temperature.

Currently, eta_cd_dimensionless_hcd_primary is calculated using:

c_hcd_primary_driven / p_hcd_primary_injected_mw

However, when no auxiliary current drive is required, PROCESS can produce:

p_hcd_primary_injected_mw = 0
c_hcd_primary_driven = 0

This causes 0 / 0, giving:

eta_cd_dimensionless_hcd_primary = NaN

eta_cd_hcd_primary is already calculated from the selected i_hcd_primary model, so eta_cd_dimensionless_hcd_primary should use eta_cd_hcd_primary directly rather than recalculating the same efficiency from realised current and power.

Proposed fix

Refactor calculate_dimensionless_current_drive_efficiency() to take eta_cd_hcd instead of c_hcd_driven and p_hcd_injected