Add pulldown to DATA_EN4

[bschulz1701]

Issue:

When the system first receives power the DATA_EN4 line (which is the control line for the Si1869 MOSFET) is high impedance since this is how the IO expander is after POR. The stray charge on this line is enough to turn on the MOSFET which control the heater power line for the Apogee port.

It is important that the heater power line not be turned on expect for SDI-12 devices. This is only a problem for 2 specific sensors in Apogee's lineup S2-131 and S2-141, but it would be very damaging to these sensors if that voltage was applied.

Potential Solution:

Tested with a pulldown (100kΩ) on the DATA_EN4 line, this is sufficient to keep the line from erroneously turning on when power is applied. This gives us time to initialize the IO expander and set this pin as a pulldown or drive it low.

After fix is made, make sure to thoroughly test that both the data line is not enabled erroneously and that the heater rail is not enabled at any point during startup, before the IO expander is able to be initialized.

Also make sure that there are no spikes to this rail after power is switched off! This was not observed when the pulldown was used, but even when the IO expander was previously configured to drive the DATA_EN4 line low, when power was switched off the heater line switched high (since the control logic voltage went away before the 12V rail discharged)