Add support for PointSources (or similar)

[BenjaminRodenberg]

In the original FEniCS adapter we support PointSources for pointwise coupling boundary conditions (e.g. forces instead of stresses). This feature currently does not exists in this adapter, but is definitely useful for certain tutorials (perpendicular flap)

[IshaanDesai]

PointSource feature is being reimplemented in dolfinx, so we have to wait for it to be done before using it in the adapter.

[BenjaminRodenberg]

I just did a quick assessment on the status of PointSources and it still looks like the feature does not exist. There is, however, a prototype provided here that we could also use as a workaround in our adapter.

[BenjaminRodenberg]

Note that #26 provides a prototypical implementation of point loads.

[NiklasVin]

In the FEniCS forum I found out that it seems like some kind of implementation of Point Sources exists in the scifem library. Maybe this could be a starting point to support Point Sources.