[python] Allow to pass optional pythonName to algorithm.load()

The used downstream generator uses an argument pythonName which seems to be only used in logging output. So far dune.generator.algorithm.load() always passes the function signature as pythonName.

When jit-compiling functions with many arguments and complicated type names, this may provide very verbose output. On the other hand, it's very well possible that in many instances of generated functions the signature coincides, while the significant difference is in the actual function body rending the verbose output useless.

This patch adds support for passing a custom value for pythonName to dune.generator.algorithm.load() by a corresponding keyword argument. If the argument is not passed, the signature is used as before.