Full support for PT15M resolution and handle overlapping timeseries

[Pluimvee]

Continued on the initial work of @wonko published in PR #196

Added support and mapping to hours for PT15M resolution including

• mixed 15 and 60 minutes resolutions (BE casus)
• multiple time-overlapping responses (DE casus)
• corner cases in which some quarters (positions) may not be given. In the below test example the red values are missing but need to be taking into account when calculating the hour avg. See test file /datasets/BE_15M_avg.xml

This is a new PR to replace #202 as I ran into github issues messing up the commitlog, files changed, etc