Support element variables in UDP output

[ngoiz]

Very minor enhancement to support element structural variables to be output in a UDP message (i.e. the CRV can now be sent).

As written out in the documentation, element variables should be included in the variables yaml file as:

        - name: 'psi'  # element variable like the CRV
          inout: 'out'
          var_type: 'node'
          position: 3  # node id
          index: 2  # index dimension

and the code will look for the relevant element and local node in which the value is stored since these variables are typically stored in [n_elem, i_local_elem, i_index] form.

OP Update:
Changed to input to be the same as for pos variables.

[codecov-commenter]

Codecov Report

Merging #160 (68168ff) into develop (536d8fd) will increase coverage by 0.00%.
The diff coverage is 70.83%.

@@           Coverage Diff            @@
##           develop     #160   +/-   ##
========================================
  Coverage    61.83%   61.84%           
========================================
  Files          123      123           
  Lines        22479    22495   +16     
========================================
+ Hits         13900    13912   +12     
- Misses        8579     8583    +4     

Impacted Files	Coverage Δ
sharpy/io/network_interface.py	74.26% <ø> (ø)
sharpy/io/inout_variables.py	53.40% <70.83%> (+1.97%)	⬆️

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[ArturoMS13]

Thank you for the contribution. I just made a very minor comment.

[ngoiz]

Many thanks for your suggestions. I have actually updated this to fetch the variable from the relevant element and local node, and the user just needs to provide the node_id and dimension index desired.