Add integral model solutions

[MuellerSeb]

• added solutions based on the effective transmissivity for the "Integral" variogram model:
  • ext_thiem_int: steady state solution
  • ext_thiem_int_3d: steady state solution incorporating vertical anisotropy
  • ext_theis_int: transient solution
  • ext_theis_int_3d: transient solution incorporating vertical anisotropy
• added fix_T_well and fix_K_well bool flag to transient heterogeneous solutions to be able to set if the well value for the effective transmissivity/conductivity should be determined from the limit (True) or from the upscaled value in the first ring segment (False, default)
  • breaking: the previous behavior was effectively this set to True, which for steep effective curves resulted in an increasing error in the effective head near the well
• updated docs (use myst parser for markdown files, only generate html and pdf)
• updated CI (fixed artifacts up-/download action, see Bump actions/download-artifact from 2 to 4.1.7 in /.github/workflows #14)
• use hatchling as build backend