Add unit tests for push-forward and pull-back operators

[yguclu]

To do

• Create unit tests for the functions in module psydac.feec.pull_push
• Create unit tests for the class Pushforward in module psydac.feec.pushforward.

Note

The Pushforward class uses the kernels in psydac.core.field_evaluation_kernels, which are tested extensively in psydac/core/tests/test_field_evaluation_kernel.py. Nevertheless, the class itself is not properly tested.