Replace theta with temperature_in_eV for the Maxwell-Juttner momentum distribution

[oshapoval]

This is a follow-up PR to #7184: it replaces Maxwell–Juttner theta / theta_function inputs with temperature_in_eV/temperature_in_eV_function with similar interface used for Maxwellian momentum distribution with temperature_in_eV . It also adds capability to read temperature_in_eV from file for Maxwell–Juttner;

User-facing interface changes:

maxwell_juttner_parser.momentum_distribution_type = "maxwell_juttner"
maxwell_juttner_parser.maxwell_juttner_temperature_in_eV_distribution_type = "parser"
maxwell_juttner_parser.temperature_in_eV_function(x,y,z) = "(1.0 + heaviside(x,0)) * m_e * clight**2 / q_e"

or

maxwell_juttner.momentum_distribution_type = "maxwell_juttner"
maxwell_juttner.maxwell_juttner_temperature_in_eV_distribution_type = "constant"
maxwell_juttner.temperature_in_eV          = m_e * clight**2 / q_e

or

maxwell_juttner_read_from_file.momentum_distribution_type = "maxwell_juttner"
maxwell_juttner_read_from_file.maxwell_juttner_temperature_in_eV_distribution_type = "read_from_file"
maxwell_juttner_read_from_file.read_temperature_in_eV_from_path = "../test_3d_initial_distribution_prepare/example-temperature-in-eV-MJ.h5"

To support both Maxwellian and Maxwell–Juttner with the same temperature_in_eV inputs, parsing and internal conversion (to either u_std or theta) were split:

• Added a shared helper parse_temperature_in_eV() used by both distributions for constant, parser, and read_from_file (Cartesian geometry + openPMD only for file).
• Always store raw temperature_in_eV (scalar / parser / openPMD field) plus the species factor m_q_e_over_mc2 = q_e / (m c²). No parse-time conversion to either u_std or theta.
• Physics conversion happens only at the consumer:
  • Maxwell–Juttner (InjectorMomentumJuttner): T_eV × q_e/(m c²)
  • Maxwellian (GetTemperatureVector): isotropic sqrt(T_eV × q_e/(m c²))

To do list:

• add option to read temperature_in_eV from openPMD file.