national read function and blend

[peterdudfield]

Detailed Description

• add read method to read national forecast fron cnn andNational_XG and blend them

Context

• https://github.com/openclimatefix/gradboost_pv/issues/58

Possible Implementation

• new read function to read both cnn and National_XG
• blend by using first 2 hours of cnn after 6 hours use National_XG. Linearily blend between them
• if any forecast is more than 2 hours old then just use the other one