Replace modbus NO_READING -9999 with less upsetting value

[neilh10]

Modbus Insitu LT500 depth sensors in the field are periodically downloaded by site hydrology crew when performing local visits. This can result in a hardcoded inband error default -9999 reading, which then causes knock on effect with the ODM2/MMW automatic scaling.
Different sensors have different ranges, and so its challenging to indicate and
Change default to value that causes less problems for the automatic scaling
For the modbus sensor hardcoded defaults are in InsituParent.cpp there are three readings
waterPressureBar - theoritically can never have negative depth use magic number -0.09876
waterTemperature - unlikely to ever indicate large negative temperature --> use -9.876
waterDepthM --> (theoritically can never have negative depth) use magic number -0.09876
Due to where sensors readings are organized this affects lib InsituModbus where readings are taken
and src/InsituParent.cpp where reading is initiated from.

Update the similar sensor KellerParent.cpp and lib KellerModbus
This is a specific sensor update, the following was listed as a system wide update 18months ago, and nothing has happened since then:
EnviroDIY#443