Add endpoints interpolate 2D processed variable

[brosaplanella]

Describe the bug
When interpolating a 2D processed variable (e.g. particle concentration) you can't obtain the endpoints (e.g. surface concentration) as it extrapolates

To Reproduce
Steps to reproduce the behaviour:

1. Run a simulation
2. Generate "Positive particle concentration [mol.m-3]" processed variable
3. Evaluate at r=0
4. You should get a NaN

Expected behaviour
Processed variables should provide boundary values in 2D as they do in 1D.