add support for `test_merge`, `text_begins` ...

[soxofaan]

python driver does not support any of the text processes yet

[soxofaan]

at first sight I thought this would be a one-liner by whitelisting these processes here:

openeo-python-driver/openeo_driver/ProcessGraphDeserializer.py

Lines 89 to 102 in ff092ee

	_OPENEO_PROCESSES_PYTHON_WHITELIST = [
	'array_apply', 'array_contains', 'array_element', 'array_filter', 'array_find', 'array_labels',
	'count', 'first', 'last', 'order', 'rearrange', 'sort',
	'between', 'eq', 'gt', 'gte', 'if', 'is_nan', 'is_nodata', 'is_valid', 'lt', 'lte', 'neq',
	'all', 'and', 'any', 'if', 'not', 'or', 'xor',
	'absolute', 'add', 'clip', 'divide', 'extrema', 'int', 'max', 'mean',
	'median', 'min', 'mod', 'multiply', 'power', 'product', 'quantiles', 'sd', 'sgn', 'sqrt',
	'subtract', 'sum', 'variance', 'e', 'pi', 'exp', 'ln', 'log',
	'ceil', 'floor', 'int', 'round',
	'arccos', 'arcosh', 'arcsin', 'arctan', 'arctan2', 'arsinh', 'artanh', 'cos', 'cosh', 'sin', 'sinh', 'tan', 'tanh',
	'all', 'any', 'count', 'first', 'last', 'max', 'mean', 'median', 'min', 'product', 'sd', 'sum', 'variance'
	]
	_add_standard_processes(process_registry_100, _OPENEO_PROCESSES_PYTHON_WHITELIST)

but it turns out that openeo-process-python project does not support these through openeo_processes.get_process.

I started a PR Open-EO/openeo-processes-python#92 to address that, but it seems that the openeo-process-python project has become a lot more complex (e.g. in terms of dependencies), since the version we currently depend on: 0.0.4 from Oct 2020 (which is latest official release on pypi at the moment).
I'm afraid that the added complexity and overhead of unnecessary/unused dependencies makes the openeo-process-python project a bit of a dead end for the python driver.

[soxofaan]

I implemented the text_ processes directly without depending on openeo-processes-python repo