Add LeakyBuildingPit element

[dbrakenhoff]

PR for LeakyBuildingPit element, plus some other developments:

Add LeakyBuildingPit elements (#72):

• add IntLeakyWallEquation mixin class
• add LeakyIntHeadDiffLineSink element for building pit with leaky walls
• add LeakyBuildingPit element, inherits from BuildingPit but adds resistance term
• add notebook testing LeakyBuildingPit by comparing to exact analytical solution for circular building pit

Add integrated normal flux method to model class (#60)

• Add ml.intnormflux() for calculating flux flowing over a specified line.
• Implement two integration methods (approximate analytical integration with legendre-quadrature and numerical intergration with scpy quad)
• Add notebook testing normflux methods.

Fix bug in resistance factor calculations (#71)

• Replace resfac = T / res by resfac = Haq / res. Tested by comparing LeakyLineDoublet solution to exact analytical solution for circular building pit.

Other changes:

• Drop Python 3.7 testing, add 3.10 and 3.11. (Drop Python 3.7 support #73)
• allow BuildingPit to be placed in models with 1 layer
• isort __init__.py
• allow passing kwargs to contour function

TODO:

• check solution
• check naming of the elements (Leaky for resistance term?, Int for integrated condition?, HeadDiff for change inside-outside?, ...)
• add tests
• documentation