Handle weightings argument to scores about scores

[tennlee]

• A weights array may be specified which would multiply pixelwise after the error is calculated and before dimensionality reduction
• The most common example of this is latitude-weighting but there are several other important use cases
• Any dimensions found in the weightings array will be assumed to be additional weighting dimensions and that a weighted score is wanted for each
• A utility function should be added to scores.utils which will generate a latitude-weighted array which can be used for a given lat/lon indexed array in LLXY projection. Other projections may be helpful but are not critical for the first implementation.
• NaNs will result in the exclusion of errors at that location from calculations (will not contribute at all rather than being zero)

Tests should include:
o Calculating a standard weightings array for a 10x10 LLXY grid and ensure weightings are as expected
o Supplying a weightings array to continuous scoring function and make sure the results are correct
o Make sure specifying a weightings array full of '1s' does not change the result compared to the ordinary scoring function for some examples
o Make sure specifying an array full of zeroes doesn't cause exceptions
o NaNs in the weights array
o A weights array consisting entirely of NaNs