Fix FutureWarnings

[maurerle]

To Reproduce The following snippet produces warnings using python 3.11 and pandas 2.2.1

from demandlib.bdew import ElecSlp
e_slp = ElecSlp(2019)

demandlib had some pandas warnings:

• FutureWarning: Downcasting object dtype arrays on .fillna, .ffill, .bfill is deprecated and will change in a future version
• FutureWarning: Setting an item of incompatible dtype is deprecated and will raise in a future error of pandas
• FutureWarning: A value is trying to be set on a copy of a DataFrame or Series through chained assignment using an inplace method

These are fixed through this commit.

[p-snft]

Hi @maurerle, thanks for your fixes (and sorry for letting this uncommented for such a long time). I just realised that demandlib needs some love (to have a maintenance release) and so I am very happy that you cared for some of the changes needed to maintain compatibility.

If you like, don't forget to add your name to the list of authors.

[pep8speaks]

Hello @maurerle! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file src/demandlib/bdew/elec_slp.py:

Line 208:1: W293 blank line contains whitespace Line 208:1: W391 blank line at end of file