New grid scripts and fix cobaya-grid-create

[cmbant]

See doc and sample input script

[codecov-commenter]

Codecov Report

Attention: 237 lines in your changes are missing coverage. Please review.

Comparison is base (7a47185) 84.82% compared to head (7b2419a) 81.72%.

Files	Patch %	Lines
cobaya/grid_tools/gridtables.py	60.36%	130 Missing :warning:
cobaya/grid_tools/gridmanage.py	86.75%	42 Missing :warning:
cobaya/grid_tools/batchjob.py	75.00%	34 Missing :warning:
cobaya/grid_tools/gridconfig.py	88.33%	14 Missing :warning:
cobaya/post.py	83.33%	5 Missing :warning:
cobaya/grid_tools/gridrun.py	86.20%	4 Missing :warning:
cobaya/grid_tools/batchjob_args.py	60.00%	2 Missing :warning:
cobaya/theories/cosmo/boltzmannbase.py	33.33%	2 Missing :warning:
cobaya/grid_tools/jobqueue.py	0.00%	1 Missing :warning:
cobaya/likelihoods/base_classes/planck_clik.py	50.00%	1 Missing :warning:
... and 2 more

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #326 +/- ## ========================================== - Coverage 84.82% 81.72% -3.11% ========================================== Files 124 133 +9 Lines 9169 11033 +1864 ========================================== + Hits 7778 9017 +1239 - Misses 1391 2016 +625 ```

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[cmbant]

@JesusTorrado test_cosmo_grid_2.yaml gives "'Varying Neff and total mass of 3 degenerate nu's'with theory code 'classy'. This does not mean that you cannot use this model with that theory code; just that we have not implemented this yet." - can we just delete this file?

[cmbant]

@JesusTorrado, if you have any thoughts on grids now might be the time to comment. I've made a first-pass attempt at supporting python grid settings files, sort-of hybrid between your dict structure and what I had in cosmomc. My current toy example is (which could be used for fast tests) is https://github.com/CobayaSampler/cobaya/blob/grid_update/tests/simple_grid.py Let me know if you have any thoughts/objections about how to best dto this.

I did have to revert some of your changes, e.g. JobItem now stores lists of parameter names and data names as before, rather than just combined name tags - this was needed to make all the job script filters (by param name, or datatag name) work, e.g. when use cobaya-grid-tables etc. (these newly imported scripts otherwise seem to all work fine with cobaya grids)

[cmbant]

@JesusTorrado OK to merge this? Most of the changes are orthogonal to the main code (and the grid code currently in master doesn't work)