WIP: Implement math operators for fieldlists

[sandorkertesz]

This now works for +, -, *, /, %, //, ** and for their 'r' versions. Unary +- are also implemented.

import earthkit.data as ekd
import numpy as np

# ds contains 2 fields with the same shape
ds = ekd.from_source("sample", "test.grib")

arr0 = [1,2]
arr1 = ds[0].values + 1
arr2 = ds.values + 1

# left operand
r = ds + ds
r = ds + ds[0]
r = ds + 1
r = ds + arr0
r = ds + arr1
r = ds + arr2
r = ds[0] + 1
r = ds[0] + arr1

# right operand
r = ds[0] + ds
r = 1 + ds

# ufunc
def func(x, y):
    return np.sin(x) + 2*y

# math methods
r = ds.sin()
r = ds[0].sin()

ds1 = ds + 1
r = ekd.utils.compute.apply_ufunc(func, ds, ds1)

TODO:

• comparison operators ?
• add control on how the resulting fields are stored. Currently all are in memory
• clarify relation to earthkit-transforms
• refactor field/fieldlist so that cloning should work for any derived field classes automatically

Problems:

In earthkit-data the + operator was already used as the concatenation operator. In Python it is not possible to add custom operators so we need to use a method e.g. concat() in the future. E.g.

# addition
r = ds1 + ds2 

# concat
r = ds1.concat(ds2)
# or
r = ekd.concat(ds1, ds2)