Use pkg-config to find muparse

Description

Currently, we use a Findmuparser module with GPL license. So, in order to proceed with #32 (closed), we must change it for another one with less restrictive license or, to find other ways to find mu parser. From inspection, I see that muparser provides a pc file.

Proposal

Require usage of pkg-config and use it in cmake to find muparser. This has the advantage that is more reliable because the pc files come defined on muparser.

How to test the implementation?

All pipelines pass, including GitHub actions

Related issues

See #32 (closed)