*TW 05/01/21*
- A couple of changes to how confusion noise is handled
- End user can now access confusion noise functions directly through `get_confusion_noise`
- Added confusion noise fit from Huang+20 to be used with TianQin
- Added confusion noise fit from Thiele+21 which is based on a WDWD population with a metallicity dependent binary fraction
- TianQin psd function now include the confusion noise
- Change defaults used in `source` and `psd`
- Often defaults for arm length, observation time and confusion noise were previously LISA related, LEGWORK now automatically works out the defaults based on what instrument is chosen
- Bug fix: in `visualisation` avoid mixing floats with Quantities when filling in a sensitivity curve