sagemathgh-36987: Doing some cleanup on Littelmann paths
    
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We do some cleanup on Littelmann paths by allowing greater input
(including covering a surprising case when passing in a Cartan type and
a weight for finite type), some PEP8 stuff, some doc formatting,
commenting out assert statements, and other misc improvements. This is
not necessarily meant to be comprehensive, but it moves things in the
right direction. The most important thing is fixing the surprising input
behavior.

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URL: sagemath#36987
Reported by: Travis Scrimshaw
Reviewer(s): Ben Salisbury

Author: Release Manager