Index for longitude roll calculated using resolution which may not be constant

[ellesmith88]

Description

In clisops.utils.dataset_utils the offset used to roll the dataset is calculated using the resolution of the data:

def calculate_offset(lon, first_element_value):
    """
    Calculate the number of elements to roll the dataset by in order to have
    longitude from within requested bounds.

    :param lon: longitude coordinate of xarray dataset.
    :param first_element_value: the value of the first element of the longitude array to roll to.
    """
    # get resolution of data
    res = lon.values[1] - lon.values[0]

    # calculate how many degrees to move by to have lon[0] of rolled subset as lower bound of request
    diff = lon.values[0] - first_element_value

    # work out how many elements to roll by to roll data by 1 degree
    index = 1 / res

    # calculate the corresponding offset needed to change data by diff
    offset = int(round(diff * index))

    return offset

The resolution is not necessarily the same throughout the dataset so we need a better solution to calculate the offset.