Refresh of calculation methods

[Pluimvee]

Code for proposal made in #194

The calculations of min, max, avg and perc should be very clear on what dataset they are performed.

With this change I tried to cleanup the calculation methods:

• today[renamed from rotation]: calculations are executed on the price set of today and changes daily
• publish: uses the dataset unfiltered. This means the calculations change whenever a new dataset is fetched
• sliding-24: instead of taking a dataset from now till the end of tomorrow it now uses a dataset of 12hrs before till 12hrs after
• sliding-12: same as above, but now using a 12hrs window (6hrs before and 6 hrs after)
• forward-24: this filter uses the upcoming 24hrs of data, not including pasted data
• forward-12: the same as above but using a 12hrs look ahead

[Roeland54]

The documentation of these methods should also be changed. You could use the content of the issue you made for this.

[Roeland54]

should we consider 'sliding-24' instead of 'sliding'? (also for forward?)

[Pluimvee]

Please review after verifying/approving/merging Pull request #201