Replace freq '1M' with '1MS'

[AdamRJensen]

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The freq='1M' is used in a couple of places in the pvlib iotools functions, which results in the following future warning:

FutureWarning: 'M' is deprecated and will be removed in a future version, please use 'ME' instead. months = pd.date_range(

The frequency 'M' is being phased out in favor of 'ME' and 'MS' which correspond to month end and month start. 'M' is equivalent to 'ME'. However, 'MS' is the only one supported in our minimum Pandas version, whereas 'ME' was first introduced in a later version. For the iotools functions, I think 'MS' is conceptually more correct.

Minimum changes to the lookup linked turbidity functions were made to make tests pass. I preferred making these modifications over calculating new test values.