Add zenith angle dependence to sea ice albedo calculations

The method for adding zenith angles to sea ice albedo calculations varies depending on the surface type:

| Snow on sea ice | Bare sea ice | Melt ponds |
| ------ | ------ | ------ |
| Use the same scheme as snow on land which already has a zenith angle dependence as well as additional improvements such as how albedo varies with snow grain size.​ | Use the same scheme as bare soil except that the albedo is shifted to represent bare sea ice.​ | Add a new scheme based on Malinka et al which explicitly represents Fresnel reflection off the surface and absorption within the pond. |

These will be added via two switches:

* Zenith angle dependence for snow on sea ice and bare sea ice is turned on via the new switch l_zenith_albedo (under '''[namelist:jules_sea_seaice]''').
* The new Malinka et al melt pond albedo scheme is turned on via a replacement switch which is replacing l_sice_meltponds_cice with i_meltpond_alb_vn (under '''[namelist:jules_sea_seaice]''') where:
* i_meltpond_alb_vn = 0 : Do not use a melt pond albedo scheme at all (equivalent to l_sice_meltponds_cice=.false.).
* i_meltpond_alb_vn = 1 : Use the original CICE melt pond albedo scheme (equivalent to l_sice_meltponds_cice=.true.).
* i_meltpond_alb_vn = 2 : Use the new Malinka et al melt pond albedo scheme.

The snow albedo scheme takes snow grain size as in input. For snow on land this is a prognostic where snow grains gradually grow over time. For snow on sea ice I have implemented a simple linear dependence between snow grain size and temperature where the snow grain size goes from snow_grain_size_min (at -30oC, and colder temperatures) to snow_grain_size_max (at 0oC). snow_grain_size_min and snow_grain_size_max are supplied via '''[namelist:jules_sea_seaice]'''.

Linked UM issue = https://github.com/MetOffice/um/issues/62
Linked lfric_apps issue = https://github.com/MetOffice/lfric_apps/issues/400

JULES branch = https://github.com/DanCopsey/jules/tree/zenith_angle_seaice

Start by reviewing the JULES branch linked in the issue and the [namelist:jules_sea_seaice] settings described here. Read the linked UM and lfric_apps issues for integration context, then verify that the zenith-angle and melt-pond switches and snow-grain-size inputs support the three stated surface types and preserve the documented legacy modes.

Written by the indexing model from the issue text.