Add efield position dependence

[mhliu0001]

Previously, only one uniform electric field was used. This PR adds position dependence to the electric field.

Another change is that I have unified the nestv2 parameter name with the other yield models. The field parameter was called "literature_field" before, and we should change it to "field" to avoid confusion.

[mhliu0001]

I need to write a test for the field dependence.

[coveralls]

Pull Request Test Coverage Report for Build 11095259874

Details

• 24 of 24 (100.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 85.347%

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Totals
Change from base Build 10709265769:	0.2%
Covered Lines:	2458
Relevant Lines:	2880

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💛 - Coveralls

[dachengx]

LGTM

1. We will need to change parameters["field"] in https://github.com/XENONnT/applefiles accordingly.
2. This PR is not for production in SR1. So it is not in a hurry to merge it now.

[dachengx]

Please generate _efield_map.json mimicking https://github.com/XENONnT/appletree/blob/88445b55f5e46dd8282b7a57c795a59a0fd8fb3c/notebooks/6.%20maps.ipynb.