Deprecate `spheroidalCF2` and `shellCF2` in `characteristicfunctions.py`

[Sparks29032]

The functions shellCF and spheroidalCF are just wrappers for shellCF2 and spheroidalCF2 respectively. Can we deprecate of of each to prevent confusion? Users reading the docs may be confused by the presence of both and not know which to use (even though they are functionally equivalent).

When deprecating, please be very careful to check across all diffpy packages where each function is being used and make those changes accordingly alongside the deprecation.

[cadenmyers13]

@sbillinge This can be closed as complete

[sbillinge]

@cadenmyers13 @Sparks29032 Please double check. This was not to deprecate camelCase but the point of this was that there were two versions of the same function that took different inputs but did the same job. We deemed that this was confusing to users and making a single function that could handle either kinds of inputs would be better.

[cadenmyers13]

@sbillinge they have been reduced down to one function. The camel case CF and CF2 variants point to a single snake case function now