Add (or readd) tests to various places

[jgostick]

• [x] models.viscosity.air_correlation
• [x] models.viscosity.gas_pure_st
• [ ] models.viscosity.gas_pure_cls
• [x] add #pragma: no cover to critical props funcs
• [x] add #pragma: no cover to models.diffusivity.gas_mixture_fw
• [x] models.vapor_pressure.liquid_pure_lk
• [x] models.vapor_pressure.liquid_pure_antoine only test the extended one, so add check for case where extended data is not found
• [x] models.thermal_conductivity.liquid_mixture_DIPPR9I
• [x] models.thermal_conductivity.liquid_pure_sr
• [x] phase.Mixture is tested, but not very thoroughly
• [ ] ~core.Base2 --> obj['conduit.diameter'] behavior is not tested~
• [x] core.Base2.keys with modes like models and constants
• [x] core.Base3.get_conduit_data has an 'edge case' that is not being covered
• [x] core.mixins.ModelsDict.str should have #pragma: no cover
• [x] In fact, let add #pragma: no cover to all print, repr and str funcs
• [x] core.mixins.ModelWrapper needs tests throughout
• [x] algorithms.drainage.pc_curve is not tested at all
• [x] algorithms.Transport._validate_data_health should be tested against a "bad" case so that all the logic get triggered
• [x] network.Network.get_incidence_matrix
• [x] network.DelaunayVoronoiDual has several added functions that are not being tested
• [x] network.Gabriel is not tested at all (consider just deleting?)

[jgostick]

• [x] skgraph and topotools need VERY thorough tests

[jgostick]

The following modules are all below 90% coverage and represent nearly a hundred lines.

• [x] models.geometry.conduit_lengths
• [x] models.geometry.throat_length
• [x] models.geometry.throat_capillary_shape_factor
• [x] models.geometry.throat_endpoints

[ma-sadeghi]

Closed in #2574