[ENH] Tidy up the utils directory

[TonyBagnall]

Describe the feature or idea you want to propose

there is a lot of old legacy code clutering up utils, this tidies it up. Some done by #1042

Describe your proposed solution

• [x] slope_and_trend.py: two private methods, slope not used anywhere, trend only used in theta forecaster. Removes slope, moved private method and test to theta.py
• [x] profiling.py: very simplistic method for time profiling a classifier. Not used or useful. Removed datetime.py: method get_time_index is only used in transformations.collections.segmenter, but _coerce_duration_to_int is used all over, leave this one
• [x] estimator_checks.py: single method called check_estimator that runs tests on a single estimator. Quite useful, but should probably be in the testing module. #1313
• [x] mflow_aeon.py. "The mlflow_aeon module provides an MLflow API for aeon forecasters." See #968 #1285
• [x] multiindex.py. Single function flatten_multiindex used in transformations and datatypes. leave
• [x] sampling.py. stratified_resample used in benchmarking/experiments. Move there
• [x] seasonality.py. autocorrelation_seasonality_test, used only in transformations.detrend.ConditionalDeasonalizer. Can move ther
• [x] sklearn.py. Tests if a sklearn estimator. USed in the overloaded operators. Not going to mess with that atm
• [ ] stats.py. Weighting functions used in performance functions and forecasting ensemble. Rename?

Describe alternatives you've considered, if relevant

No response

Additional context

No response

[itsdivya1309]

Hi! I would like to work on the sampling.py and seasonality.py files.

[TonyBagnall]

all done