Replace CONCENTRATION_*** units with corresponding enum(s)

[epenet]

We currently have 6 concentration units which would be nice to migrate to UnitOf constants.

Mass over Volume:

• CONCENTRATION_MICROGRAMS_PER_CUBIC_METER: Final = "µg/m³"
• CONCENTRATION_MILLIGRAMS_PER_CUBIC_METER: Final = "mg/m³"
• CONCENTRATION_MICROGRAMS_PER_CUBIC_FOOT: Final = "μg/ft³"

Parts over volume:

• CONCENTRATION_PARTS_PER_CUBIC_METER: Final = "p/m³"

Parts over parts:

• CONCENTRATION_PARTS_PER_MILLION: Final = "ppm"
• CONCENTRATION_PARTS_PER_BILLION: Final = "ppb"

Partially linked to #651 and #1146

[epenet]

One option would be to add all of them under a generic UnitOfConcentration:

• UnitOfConcentration.MICROGRAMS_PER_CUBIC_METER
• UnitOfConcentration.MILLIGRAMS_PER_CUBIC_METER
• UnitOfConcentration.MICROGRAMS_PER_CUBIC_FOOT
• UnitOfConcentration.PARTS_PER_CUBIC_METER
• UnitOfConcentration.PARTS_PER_MILLION
• UnitOfConcentration.PARTS_PER_BILLION

[epenet]

Another option would be to treat them as three groups:

• add an enum for mass over volume units to handle µg/m³, mg/m³, μg/ft³ (eg. UnitOfMassDensity, or UnitOfDensity, ...)
• add an enum for parts over volume units to handle p/m³
• add an enum for unit-less ratios to handle ppm and ppb (and maybe also %)

[epenet]

Suggested implementation for this solution is proposed under home-assistant/core#129336 + home-assistant/core#129337
Parts over volume seems to only be used by accuweather so I'm not sure if it should even be kept in const.py

[epenet]

Another option would be to treat them as two groups:

• add an enum for "anything" over volume units to handle µg/m³, mg/m³, μg/ft³, p/m³ (eg. UnitOfVolumeConcentration)
• add an enum for unit-less ratios to handle ppm and ppb (and maybe also %)