what is done for ellipse arcs in the built-in kernel. Fixes #578.

Interface OCC sewing algorithm in the creation of surface loops, with extra arg in API or "Using Sewing" keywords in .geo files (fixes #583)

new mesh::clear()

Rationale: setNodes/setElements were intrinsically a bad design choice,
as these functions had to delete nodes and/or elements before storing the
new ones. Using these functions in the wrong way could lead to an invalid
mesh, with elements pointing to nodes that were deleted (in an adjacent
entity).

The only viable workflow is to delete the mesh (with the new clear()
function), before adding nodes/elements.

This reverts commit 07cc55d9

This reverts commit 211fa521, reversing
changes made to 8ba56d73.

modify getJacobians, getBasisFunctions & co to take integrationPoints directly as argument (instead of integrationType)

- document getBoundingBox when dim and tag are < 0

don't issue error if mesh of invisible region contains no elements while Mesh.MeshOnlyVisible is set (fixes #532)

- import STL solid names