`BaseLammpsCalculation`: Add the `script` input

[sphuber]

Fixes #55

This allows a user to pass a complete input script that should be used as-is. The structure, potential and parameters inputs are completely optional in this case. This gives ultimate flexibility to a user to run whatever they want in exchange for queryability.

[codecov[bot]]

Codecov Report

Merging #59 (fd8a395) into develop (d679df5) will increase coverage by 0.03%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop      #59      +/-   ##
===========================================
+ Coverage    90.24%   90.27%   +0.03%     
===========================================
  Files           32       32              
  Lines         2408     2416       +8     
===========================================
+ Hits          2173     2181       +8     
  Misses         235      235              

Flag	Coverage Δ
pytests	90.27% <100.00%> (+0.03%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
aiida_lammps/calculations/lammps/base.py	96.66% <100.00%> (+0.32%)	:arrow_up:

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[JPchico]

This looks good @sphuber ! Sorry for the delay in approving it, I have been sick the entire last week.

[sphuber]

No problem, thanks for the review!