Avoid direct dependence on `earthkit-meteo`

[Oisin-M]

What happened?

We currently require earthkit-meteo as an non-optional dependency for earthkit data, even though it is barely used i.e. only here:

earthkit-data/src/earthkit/data/sources/forcings.py

Lines 182 to 204 in 891e2e5

	def toa_incident_solar_radiation(self, date):
	from earthkit.meteo.solar import toa_incident_solar_radiation
	date = to_datetime(date)
	result = toa_incident_solar_radiation(
	date - datetime.timedelta(minutes=30),
	date + datetime.timedelta(minutes=30),
	self.latitude_(),
	self.longitude_(),
	intervals_per_hour=2,
	)
	return result.flatten()
	def cos_solar_zenith_angle(self, date):
	from earthkit.meteo.solar import cos_solar_zenith_angle
	date = to_datetime(date)
	result = cos_solar_zenith_angle(
	date,
	self.latitude_(),
	self.longitude_(),
	)
	return result.flatten()

It would be better to have it optional in order to avoid any circular dependency issues in future should earthkit-meteo require functionality from earthkit-data.

What are the steps to reproduce the bug?

Version

latest

Platform (OS and architecture)

any

Relevant log output

Accompanying data

No response

Organisation

No response