Sfsimodels

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* Added `Coords` object, which defines coordinates in x, y, z directions
* Added `Units` and `GlobalUnits`
* Added `Load` and `LoadAtCoords` objects
* Better loading of ecp files - can now deal with non-defaults kwargs, e.g. changing `liq_mass_density` in a Soil object
* Added method for computing column vertical loads (`get_column_vert_loads`) to Frame object
* Added PadFooting object, and PadFoundation now as a PadFooting object to store pad footing attributes

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* Added option to provide shear modulus at zero effective stress for stress dependent soil.
* Can load ecp file and if model not defined then can set flag to load the `base_type` model.
* Added mesh generation tool for creating meshes for finite-element analyses based on new TwoDSystem object.
* Added `NullBuilding` object to create a building with no attributes
* Added option for linking a building and a foundation using `building.set_foundation(foundation, two_way=True)` and `foundation.set_building(building)`, where if `two_way` is true then vice-versa link also created.
* Fixed issue were checking_tools would raise error for zero vs almost zero.

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* Can now pass `export_none` to ecp_object.add_to_dict() to turn on/off the export of null in ecp file.
* Can assign beam and column properties to frames using 'all' to assign to all beams or columns.

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* Fixed issue with stack not working for overriding soil properties
* Switched to using wheel distributions on pypi for package

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* Can now explicitly set gravity for soil (default=9.8) and liquid mass density `liquid_mass_density` (default=1000)
* `pw` is deprecated but still available and replaced with new name `uww` due to poor name