Pre-exponential factor dependent of the species

[ehodille]

One can use some bit of theory to assume that the pre-exponential factor of isotope A is
$D_{A,0} = \frac{D_{\mathrm{H},0}}{\sqrt{\frac{m_A}{m_\mathrm{H}}}}$
Thus, if one want to use it in multi isotope modes, the following line
https://github.com/kaelyndunnell/hisp/blob/943830204fb89aa5e0dcdbf269aedfabf6515da6/src/hisp/festim_models/mb_model.py#L68

would probably change to something like:
D_0 = { 'D':w_diffusivity.pre_exp.magnitude/np.sqrt(2), 'T':w_diffusivity.pre_exp.magnitude/np.sqrt(3) }
If I understand well how the material class works here (https://github.com/festim-dev/FESTIM/blob/939540b09c46f4ec8d231ff8264f799182ad1610/src/festim/material.py#L6-L47)

Similar thing can be expected here https://github.com/kaelyndunnell/hisp/blob/943830204fb89aa5e0dcdbf269aedfabf6515da6/src/hisp/festim_models/mb_model.py#L330 and here https://github.com/kaelyndunnell/hisp/blob/943830204fb89aa5e0dcdbf269aedfabf6515da6/src/hisp/festim_models/mb_model.py#L599

Also, the trapping and detrapping rate for all materials can be impacted
It would already be taken into account for the trapping in tungsten as it depends on D_0
https://github.com/kaelyndunnell/hisp/blob/943830204fb89aa5e0dcdbf269aedfabf6515da6/src/hisp/festim_models/mb_model.py#L132
maybe this line should be changed to
k_0=D_0['D'] / (interstitial_distance**2 * interstitial_sites_per_atom * w_density), for deuterium or D_0['T'] for tritium