correct units notation in headers

@mschleiss wrote:

The notations for the units in the column headers of SVFS files are inconsistent. For example, "Rain.intensity..mm.h." should denote mm/h. But "Kinetic.energy..J..m.2h.." should be J/(m^2*h). Some clear symbols and/or conventions for divisions, multiplications and power operators in units are needed. Similarly, Snow.depth.intensity..mm.h." should be mm/h.

The way to address the issue is to

• define clear symbols and/or conventions for divisions, multiplications and power operators
• using the parsivel documentation p.29 adapt the units notation in pdf to convention symbols and edit the units in https://gitlab.tudelft.nl/ruisdael/disdrodlv2/-/blob/main/config_GV_008.yml#L11