Fix hydropower and load bugs

[joph]

Changes proposed in this Pull Request

Bugfix for approximate_missing_eia_stats()

If ES or PT were not present in the list of optimized countries, the IEA hydropower approximation would fail, i.e. approximate_missing_eia_stats(). Therefore runs, where the option eia_approximate_missing was activated would fail for weather years not part of the IEA data. The functoin would throw a key error:
KeyError: "None of [Index(['ES', 'PT'], dtype='object', name='name')] are in the [index]"

The reason is that approximate_missing_eia_stats() overwrites anomalous years for 'ES' and 'PT'. By checking if the countries are present before overwriting the data, the bug can be fixed:
if ("ES") in runoff:
runoff.loc[1978, ["ES"]] = runoff.loc[1979, ["ES"]]
if ("PT") in runoff:
runoff.loc[1978, ["PT"]] = runoff.loc[1979, ["PT"]]

Bugfix for manual_adjustment()

The copy_timeslice() procedure will add countries to the load data which are not present in the countries list, if a fn_load object is provided. If manual_adjustment() is called, which uses copy_timeslice(), this will partly add data for some countries such as GB. Later on, this causes an AssertionError: Load data contains nans.

Checklist

• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to envs/environment.yaml.
• Changes in configuration options are added in all of config.default.yaml.
• Changes in configuration options are also documented in doc/configtables/*.csv.
• A release note doc/release_notes.rst is added.

[fneum]

Thanks @joph! Looks good with a minor style adjustment.