`air_particulate_matter_concentration` -> `air_PM_concen` -> `air_pm_concen`

[turbomam]

for the Air extension

In version 6.0.0, this slot was called air_particulate_matter_concentration

That was shortened to air_PM_concen at soem point during the evolutio of the version 6 family, but that violates our slot naming policy (lower_snake_case only)

Since it has only be in MIxS as air_PM_concen for a short time, I suggest we change it to air_pm_concen now in preparation for version 7

[turbomam]

Neither air_particulate_matter_concentration nor air_PM_concen appear in the INSDC Biosamples

[lschriml]

To be reviewed by CIG.