v1.15.0

• Minimum-lengthscale filters for adjoint optimization (#1205).

• Python API documentation in docstrings (#1240).

• MaterialGrid material type in Python to interpolate an array of material
  values as the "material" of an object, especially for topology optimization (#1242).

• merge_subgroup_data Python function for coordinating parallel
  computations (#1192).

• Eigenmode sources now ensure that the source has the same
  frequency as the mode (#1218).

• Performance improvements to eigenmode sources (#1233, #1244, #1257).