conversionPartLoad subclass: discretization variables spelling inconsistency

[d00958]

Hi all,
in my recent experiments with the conversion subclasses, I have come across what appears to be a small inconsistency in the conversionPartLoad subclass.

On a side note: the discretization variables discretizationPointVariablesOptimun, discretizationSegmentConVariablesOptimun, and discretizationSegmentBinVariablesOptimun are all spelled with an "n" at the end. This is not what I am referring to as this is fully consistent within the subclass and does not cause an issue.

However, when calling writeOptimizationOutputToExcel(), I got an error saying:

AttributeError: 'ConversionPartLoadModel' object has no attribute '_discretizationPointVariablesOptimun'

As it turns out, the aforementioned discretization variables are all initialized and set without an underscore, but getOptimalValues() tries to retrieve them as _discretizationPointVariablesOptimun. Eliminating the underscore seems to have solved this for me.

I am not sure if this is relevant or if conversionPartLoad plays a role in further development, but maybe someone else faces the same. Also, an option to combine conversionPartLoad and conversionDynamic would be a dream come true (if even mathematically feasible).

[d00958]

For the sake of completeness, sheet names also need to be truncated for conversionPartLoad. See also discussion #58 & issue #59