Add/Edit Variables and Units in Database

[PR-HL]

1. Current variable list requires an option to edit the units of existing variables.

• Database format : "volumetric water content (cm3/cm3), rainfall sum (mm/day)
• ISMN format : "soil moisture content (m3/m3 * 100), precipitation (mm)

2. Would be good to have an option to add new variables.

[flofunk]

I have the same problem with these three variables:

1. the variable "bulk electrical conductivity" has the unit "dS/cm" in the uploader. The data I have has the unit "dS/m" though.
   
2. the variable "ground water level" needs to be added with the unit "mm"
3. the variable "logger temperature" needs to be added with the unit "°C"