[JOSS] Revise statement of need #119

The current statement of need does not live up the JOSS requirements:

A section that clearly illustrates the research purpose of the software and places it in the context of related work. This should clearly state what problems the software is designed to solve, who the target audience is, and its relation to other work.

The current section focuses too much on climate change and why PV is important, whereas the focus should be on what problem the tool is trying to solve and target audience.