Add drop_na() method to Series class

[niatimary]

• Adds Series.drop_na() method to remove NaN values while preserving temporal order, complementing the existing Series.clean() method which removes NaNs and sorts data.
• Add TestUISeriesClean.test_drop_na() in pyleoclim/tests/test_core_Series.py

[CommonClimate]

Please:

• rename to dropna()
• reuse tsbase.dropna() (see init_ for an example of how this is one)
• this seems overkill:

time = ts_clean.__dict__["time"]
 value = ts_clean.__dict__["value"]

you should be able to access them by ts_clean.time and ts_clean.value. AI coding is not always best...

[CommonClimate]

You can simply re-use this bit:

 if dropna == True:
            value, time = tsbase.dropna(value, time, verbose=verbose)
            if keep_log == True:
                if len(self.log) > 0:
                    if self.log[0][0] == 'dropna' and self.log[0]['applied'] == True:
                        pass # no need to clog the log with redundant information
                    elif self.log[0][0] == 'clean_ts' and self.log[0]['applied']==True:
                        self.log[0]['legacy']=True
                        self.log += ({len(self.log): 'dropna', 'applied': dropna, 'verbose': verbose},)
                else:
                    self.log += ({len(self.log): 'dropna', 'applied': dropna, 'verbose': verbose},)
        elif dropna == False:
            pass
        else:
            raise ValueError('dropna should be a boolean')