Support configuring trigger slope in nidmm.

[l-johnston]

In LabVIEW it is possible to set the trigger slope using niDMM Configure Trigger Slope.vi, but in Python, there is no such function or property. I tried the following workaround that uses the private _set_attribute function, but this did not work - the hardware continues triggering on the rising edge which is the default for the PXIe-4081.

session._set_attribute_vi_int32(NIDMM_ATTR_TRIGGER_SLOPE, NIDMM_VAL_NEG)

[ni-jfitzger]

This was removed in #875.
I'm not sure about the bit about the private call not working. Your code's a little C-ish. A better way to write it is shown here. But I would expect that to work (after a commit) unless it's not supported by the hardware.