v0.35 release candidate: GroPoint Modbus; Xbee & Digi WiFi fixes; Prep for batch transmission #463

For Modbus GPLP-8 sensor only, for now.

...replacing any vestiges from `YosemitechY4000.h`

from GroPointProfileGPLP8

Correctly initialize response code variable terminator.

Saves 750 bytes of flash as the buffer can be placed in .bss to be zero-initialized instead of .data.

Saves ~110 bytes of flash and substantial stack

Saves ~200 bytes of RAM and ~360 bytes of flash. The equations reproduce the tables previously found in the source code exactly. The reason for the values in the original tables is unknown.

The test for "15 seconds before the next logging interval" has been wrong for years, possibly since this code was written, with no apparent consequence. The behavior is additionally confusing to users deploying the devices and causes problems with logging as the modem won't get turned off for a long time. Remove it completely to solve the problems.

Use cleaner interface and common functions that avoid repeated snprintf and strlen usage to save ~2.5KB of flash and dozens of lines of code. Removes extra \r\n from HTTP requests as a side effect, which were against spec and caused spurious 400 Bad Request status messages from servers.

sendEveryX will be used in the future for data buffering functionality. Increase its width to an int to allow larger buffers when desired. Delete sendOffset completely as there is little reason for that particular functionality. The offset will be in effect set randomly using the time the datalogger initially powers on.

The C++ standard specifies that all objects in the same translation unit (i.e. source file) are constructed in order of declaration. Since this is the most common case when using Modular Sensors, the described case cannot occur.

Everything is working well!

Depends on https://github.com/EnviroDIY/GroPointModbus

Replace UNKNOWN w/ GPLPX for `gropointModel`, because it conflicted with YosemiTechModbus `UNKNOWN`