Rename path_length attribute of Path and Surface classes

[jobdewitte]

Problem statetment

Right now the class attribute of the Path and Surface class in the geometry module that represent the length of the parameter intervals in the 1D parameter space are chararacterized as path_length . This is easily confused with the length of the resulting path in the 3D target space and will likely lead to inconsistencies in further development.

Proposed Solution

Rename the path_length attribute to something more accurate, such as param_range, _size or _extent and reserve path_length for the length of the curve in target space.

[jobdewitte]

This was a bit hasty. I'll come back to it later.

[jobdewitte]

What should happen is that the parametrization should no longer always be [0, path_length] but rather [0, u_max]. This is both advantagous from a UX and development perspective. For instance, it is much more natural to parametrize angles using [0,2pi]. However, path_length is used in Mesher, so we have to compensate for that. @leon-vv thoughts?