Fix checking against existing MATH parameters

[germa89]

The parameter check should be done on the capital version of the parameter name:

if name.upper() not in self._parm:
    self._mapdl.run(f"*VEC,{name},{MYCTYPE[dtype]},ALLOC,{size}", mute=True)

[codecov]

Codecov Report

Merging #1537 (fa4bd80) into main (5229491) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1537   +/-   ##
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  Coverage   81.80%   81.80%           
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  Files          43       43           
  Lines        6797     6797           
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  Hits         5560     5560           
  Misses       1237     1237