Plum dispatch>2.3.0 may cause compatibility issue with PySAGES

[yzjin]

A commit in the plum dispatch 2.3.0 contains a change that is not well documented. The Dispatcher.methods attribute is no longer a list of signature, but become a list of Method, which pushed the signature to one level below and cause compatibility issues with pysages. The has_method() function here will no longer be able to fetch the signature of the method correctly. I will create a pull request later to address this issue.

[pabloferz]

Closed by #324