Enable to use forcings without providing a source

[sandorkertesz]

With this PR it is possible to create "forcings" providing latitudes and longitudes only:

import earthkit.data as ekd
ds = ekd.from_source( "forcings", 
        latitudes=latitudes, 
        longitudes=longitudes, 
        date=date, 
        param=param)

The kwargs can also be "latitude" and "longitude".

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.65217% with 4 lines in your changes missing coverage. Please review.

Project coverage is 90.76%. Comparing base (a2804c0) to head (0b26d64).

Files with missing lines	Patch %	Lines
tests/forcings/forcings_fixtures.py	80.00%	1 Missing and 1 partial :warning:
tests/forcings/test_forcings_proc.py	88.88%	1 Missing :warning:
tests/forcings/test_forcings_slice.py	94.44%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #495 +/- ## =========================================== + Coverage 90.73% 90.76% +0.03% =========================================== Files 144 144 Lines 9896 9956 +60 Branches 473 476 +3 =========================================== + Hits 8979 9037 +58 - Misses 761 762 +1 - Partials 156 157 +1 ```

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