[com1DFA] Add TJEM entrainment model

[Michael-Pallua]

The goal is to make the TJEM entrainment model, which is already present in com9MoTVoellmy, available in com1DFA.

Short description of changed files:

• com1DFACfg.ini:
  A parameter to select the desired entrainment model (etnrType) is included as well as the parameter of the new entrainment model: the shear strength of the erodible snow cover (entShearStrength).

• com1DFA.py:
  Changes are made to implement the new selection between the two now available entrainment models.

• DFAfunctionsCython.pyx and DFAfunctionsCython.pxd
  A new function, computeEntMassAndForceTjem(), which implements the equations of the TJEM entrainment model is added.

PR Checklist

Please confirm before requesting review:

• I ran pytest locally without fails
• I added/updated tests where needed
• I updated documentation where needed

Confirm before the final merge/rebase into master

• Commits are sensibly squashed and rebased onto latest master
• Standardtest run without difference (with recompiled cython code)

[qltysh]

All good ✅