Bugfix - (some) bindings not working for complex type scalar

**Short description** Multiple instances of pybind throwing an error in a complex basis, where it does not for a real-type scalar basis. The error is of the type `Unregistered type : ...<std::complex<double> >` and appears for bindings implemented in `interfaces.hpp`. **Related issues** #1089 **To do** - [x] isolate root of issue - [ ] fix issue (such that complex type scalar GQCP objects are also "registered" to pybind)