Revise API for component parameters, constants, and states

[ThibHlln]

In Component definition (relevant for contributors):

• add compulsory 'default_value' item to replace default value in signature of component run method

Old API

class MyComponent(...):

    _constants_info = {
        'constant_name': {
            'units': 'm2'
        }
    }

    def run(..., constant_name=1., **kwargs):
        ...

New API

class MyComponent(...):

    _constants_info = {
        'constant_name': {
            'units': 'm2',
            'default_value': 1.
        }
    }

    def run(..., constant_name, **kwargs):
        ...

• add support for multiple divisions for component states, including divisions using component constants allowing for user-customisable divisions if they adjust the constant default_value, which resolves #31

class MyComponent(...):

    _states_info = {
        'state_name': {
            'units': 'kg m-2',
            'divisions': (7, 'constant_name')
        }
    }
    _constants_info = {
        'constant_name': {
            'units': '1',
            'default_value': 10
        }
    }

In Component implementation (relevant for contributors):

• give parameter/constant values to component initialise/finalise methods, which can be useful to determine initial conditions for component states, e.g. if one wants to start with soil layers half full, knowing the soil layer depths can be useful, or if one wants to initialise top soil layers differently from lower soil layers, knowing the number of divisions for soil layers can be useful too

In Component instantiation (relevant for users):

• add units requirement for parameter/constant values (either using a cf.Data with units or using a pair [data, units])
• add support for non-scalar parameter values (using a cf.Field, a cf.Data, or a pair [numpy.array, units]), which resolves #21

Old API

parameters = {
    'parameter_name': 1.
}

New API

parameters = {
    'parameter_name_1': [1., 'm'],
    'parameter_name_2': cf.Data(1., 'm'),
    'parameter_name_3': (numpy.ones((3, 2), 'm'),
    'parameter_name_4': cf.Data(numpy.ones((3, 2), 'm'),
    'parameter_name_5': cf.Field(...)
}