PointValue quantity in FESTIM2

We would need the equivalent of `PointValue` in FESTIM2. This should be sufficient:

```python
import festim as F

from dolfinx.geometry import bb_tree, compute_colliding_cells, compute_collisions_points

class PointValue(F.VolumeQuantity):
def __init__(self, field, volume, x0, filename = None):
super().__init__(field, volume, filename)
self.x0 = x0

def compute(self):
u = self.field.solution
mesh = u.function_space.mesh
tree = bb_tree(mesh, mesh.geometry.dim)
cell_candidates = compute_collisions_points(tree, self.x0)
cell = compute_colliding_cells(mesh, cell_candidates, self.x0).array
assert len(cell) > 0
first_cell = cell[0]

self.value = self.field.solution.eval(self.x0, first_cell)
self.data.append(self.value)
```