Add `in` Operator for `DataClass`

[jianyangli]

This is a request for

• a new feature
• an enhancement to existing sbpy functionality
• somethings else: [explain here]

The requested changes will be implemented by

• me
• the sbpy developers

High-level concept
sbpy.DataClass allows aliases for column names for flexibility. A in operator would be useful to extend the flexibility for the operations on fields.

Explain the relevance to sbpy
New feature to allow for better operability of sbpy.DataClass.

Proposal details
Implement DataClass.__contains__ method to allow for in operator. Make use of DataClass._translate_columns method.

Example (pseudo-)code

from astropy.time import Time
from sbpy.data import Ephem

ceres = Ephem.from_horizons('ceres', epochs=Time('2022-01-01'))
print(ceres['rh'])  # print [2.7183022] AU
print(ceres['r'])  # print [2.7183022] AU
print('r' in eph.field_names)  # print True
print('rh' in eph.field_names)  # print False

# new feature will allow for both lines below to print `True`
print('r' in eph)
print('rh' in eph)  # print False, but I hope this will print True