Add min/max temperature retrieval in Historian class
Add min/max humidity retrieval in Historian class
Add min/max pressure retrieval in Historian class
Add min/max rain retrieval in Historian class and filtering for None values in sample series
Add average methods
Add unit of measure in min,max,average methods for temperature series
Introduce checks on min/max/average-ing on empty data series
Merge pull request #30 from csparpa/feature/#19_historian_addons

Feature/#19 historian addons