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

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Additional details and impacted files

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