Iss #304 speedsort fix variable names

[BiancaMorandi]

Issue #304

• solved issue when same windspeed or same direction are given as the reference and the target..issue solved for all correlation functions as fix done for CorrelBase
• added general function in utils to _rename_equal_elements_between_two_inputs
• fixed issue for MultipleLinearRegression function. The original pd.Series names were changed when the list of input reference pd.Series had the same name. Fixed the code to change the name if only pd.Series used in the function.
• added tests for this and added test_utils.py file
  Note that 2 tests for speedSort synthesize are giving an error due to issue [average_data_by_period] Bug on wind direction average derivation #319

[BiancaMorandi]

as for Stephen's review to do:

• move if statements inside CorrelBase into new function
• rename all input1 names adding suffix (not only the duplicate one) - _rename_equal_elements_between_two_inputs
• remove default None to input1_suffix inside _rename_equal_elements_between_two_inputs function
• compare only lists and not str (can accept str but then it is converted to list of 1 element) - _rename_equal_elements_between_two_inputs

[BiancaMorandi]

@stephenholleran I made the changes you have requested. Happy to discuss.

[BiancaMorandi]

• Move _rename_equal_elements_between_two_inputs inside correlation.py
• remove all errors raised
• move convert string to list to function inside CorreBase
• remove all tests in utils

[BiancaMorandi]

@stephenholleran I made all the updates you have requested. The test_correlation.py should raise two errors. This is normal as the errors are due to issue #319