Metrics not to assume on pd.Series name

[MartinoMensio]

Hey,
Thank you for the amazing features of this library. I found some limitations of the implemented metrics:

• some of the metrics do not work if columns are named differently

I could rename columns before passing to the affected metrics, but it would be nicer if the functions don't rely on a column to be specifically named "load".

Example:

import pandas as pd
from openstef.metrics import metrics

df = pd.DataFrame(
    {
        "a_ground_truth": [1, 2, 3, 4],
        "a_forecast": [1.1, 2.1, 3.1, 4.1],
        "b_ground_truth": [1, 2, 3, 4],
        "b_forecast": [1.1, 2.1, 3.1, 4.1],
    }
)

a_r_mne_highest = metrics.r_mne_highest(df["a_ground_truth"], df["a_forecast"])
b_r_mne_highest = metrics.r_mne_highest(df["b_ground_truth"], df["b_forecast"])

print(a_r_mne_highest, b_r_mne_highest)

Trace of the exception:

Traceback (most recent call last):
  File "/home/martino/.cache/pypoetry/virtualenvs/load-forecasting-8OKFhNKY-py3.10/lib/python3.10/site-packages/pandas/core/indexes/base.py", line 3805, in get_loc
    return self._engine.get_loc(casted_key)
  File "index.pyx", line 167, in pandas._libs.index.IndexEngine.get_loc
  File "index.pyx", line 196, in pandas._libs.index.IndexEngine.get_loc
  File "pandas/_libs/hashtable_class_helper.pxi", line 7081, in pandas._libs.hashtable.PyObjectHashTable.get_item
  File "pandas/_libs/hashtable_class_helper.pxi", line 7089, in pandas._libs.hashtable.PyObjectHashTable.get_item
KeyError: 'load'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/martino/code/energy-forecast/load-forecasting/example.py", line 13, in <module>
    a_r_mne_highest = metrics.r_mne_highest(df["a_ground_truth"], df["a_forecast"])
  File "/home/martino/.cache/pypoetry/virtualenvs/load-forecasting-8OKFhNKY-py3.10/lib/python3.10/site-packages/openstef/metrics/metrics.py", line 175, in r_mne_highest
    if (combined["load"].max() - combined["load"].min()) != 0
  File "/home/martino/.cache/pypoetry/virtualenvs/load-forecasting-8OKFhNKY-py3.10/lib/python3.10/site-packages/pandas/core/frame.py", line 4102, in __getitem__
    indexer = self.columns.get_loc(key)
  File "/home/martino/.cache/pypoetry/virtualenvs/load-forecasting-8OKFhNKY-py3.10/lib/python3.10/site-packages/pandas/core/indexes/base.py", line 3812, in get_loc
    raise KeyError(key) from err
KeyError: 'load'

[Jonasvdbo]

@bartpleiter and @daanvanes, could you please provide an update on the status of this PR? It seems to have been open for quite some time now.