Move constants from availability.py to constants.py

[clmould]

These constants are currently in availability.py:

PROCESS/process/models/availability.py

Lines 53 to 60 in feb7ef3

	DAY_SECONDS = 60 * 60 * 24
	# Number of seconds in a day [s]
	DAYS_IN_YEAR = 365.25
	# Number of days in a year
	YEAR_SECONDS = DAY_SECONDS * DAYS_IN_YEAR
	# Number of seconds in a year [s]

They should be moved to constants.py

[timothy-nunn]

What do we think about other use of constants?

PROCESS/process/models/tfcoil/quench.py

Lines 18 to 19 in feb7ef3

	COPPER_DENSITY = 8960.0 # [kg/m³]
	NB3SN_DENSITY = 8040.0 # [kg/m³]

PROCESS/process/models/engineering/materials.py

Line 10 in feb7ef3

poisson_steel: float = 0.3

[clmould]

What do we think about other use of constants?

PROCESS/process/models/tfcoil/quench.py

Lines 18 to 19 in feb7ef3

	COPPER_DENSITY = 8960.0 # [kg/m³]
	NB3SN_DENSITY = 8040.0 # [kg/m³]

PROCESS/process/models/engineering/materials.py

Line 10 in feb7ef3

poisson_steel: float = 0.3

@timothy-nunn
I think they should all go into constants.py, I can add those into this issue ? Just so they're all contained in one location

Related to this - I did make an issue before (#4388) to ask if we should have more specific constants files, so keen to get opinions on that!