Update tests and examples so that they are no longer using "low precision theories"

[scarlehoff]

Before 4.0.9 theories were very big and fitting or loading them was very slow, and so "low precision" theories were used for the tests.

This is no longer needed (as long as only a subset of theories are used), all tests should use standard theories, i.e., all NNLO tests should use 40_000_000, all NLO and QED tests the corresponding versions etc.

This task is not particularly complicated and will make the tests much easier to maintain. In addition, no old theory should be used any longer.

1. Create a version of all fits used in the tests (https://github.com/NNPDF/nnpdf/blob/master/validphys2/src/validphys/tests/conftest.py#L75) with the corresponding theory (most of them 40_000_000)
2. Remove THEORYID=162 and THEORYID_NEW=399 from that same file. Update all tests to use THEORYID=40_000_000
3. Run the test and fix any remaining problems (e.g., the regression tests will surely need to be updated, cfactors might be different, etc)
4. Do the same with the examples in the validphys test folder https://github.com/NNPDF/nnpdf/tree/master/validphys2/examples make sure they all still run!
5. Update the documentation to no longer talk about using low precision theories.