Fix and improve TPC Loopers implementation

[jackal1-66]

• Moved the TPCLoopers inclusion to Generator.cxx
• Made forward declaration for GenTPCLoopers
• Switched to normal pointer instead of unique_ptr (destroyed in Generator destructor) ==> moved outside ifdef scope, this was the main cause of the segmentation issue when using the external generators, such as Update tune from SCCR to QCDCR for pO and OO O2DPG#2256

[github-actions]

REQUEST FOR PRODUCTION RELEASES:
To request your PR to be included in production software, please add the corresponding labels called "async-" to your PR. Add the labels directly (if you have the permissions) or add a comment of the form (note that labels are separated by a ",")

+async-label <label1>, <label2>, !<label3> ...

This will add <label1> and <label2> and removes <label3>.

The following labels are available
async-2023-pbpb-apass4
async-2023-pp-apass4
async-2024-pp-apass1
async-2022-pp-apass7
async-2024-pp-cpass0
async-2024-PbPb-apass1
async-2024-ppRef-apass1
async-2024-PbPb-apass2
async-2023-PbPb-apass5

[sawenzel]

better to check if the pointer is actually not null before deleting