Major change in behaviour of settings

[n-claes]

PR description

This is the first pull request in a series of refactoring and behaviour change PR's dedicated to Legolas 2.0.

Here a new settings object (called settings) is created to handle the global program settings and state. Most of the global variables have been removed from mod_global_variables except a few parameters and moved to this new object. Consequently, this also avoids changing global variables during runtime execution as in most routines the settings object is passes with intent(in) only. Dedicated getters and setters for set-once, use-only parameters further solidify this.

Note that this PR breaks the Legolas API in the user submodule. For example, enabling resistivity in the user submodule:

! before
resistivity = .true.
use_fixed_resistivity = .true.
fixed_eta_value = 0.0001d0

With the new changes, this becomes more logical and user-friendly:

! after
call settings%enable_resistivity(fixed_resistivity_value=0.0001d0)

with similar behaviour for other physics. See settings/mod_settings for the public API.

The datfile remains unchanged and is still backwards compatible. Some deprecated parfile settings have been removed, the docs will be updated accordingly when Legolas 2.0 is ready.

New features

Legolas

• Removal of most global variables
• Introduction of new settings object to control program settings and state
• More logical default behaviour and settings control through the user submodule

[codecov]

Codecov Report

Merging #114 (03d273d) into develop (a85c5a8) will increase coverage by 0.44%.
The diff coverage is 92.38%.

❗ Current head 03d273d differs from pull request most recent head 0977beb. Consider uploading reports for the commit 0977beb to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #114      +/-   ##
===========================================
+ Coverage    88.94%   89.39%   +0.44%     
===========================================
  Files          129      143      +14     
  Lines         7516     8098     +582     
===========================================
+ Hits          6685     7239     +554     
- Misses         831      859      +28     

Flag	Coverage Δ
legolas	90.40% <92.38%> (+0.01%)	⬆️
pylbo	87.46% <ø> (+0.92%)	⬆️

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