splitting LpSeminorm/Basic

Stream: mathlib4

[Etienne Marion]

Following #mathlib4 > long files, I took a look at file#MeasureTheory/Function/LpSeminorm. It does not seem obvious to me how to split it, but I thought of moving the results about eLpNorm in one file and in an other the results about Memℒp. However those are mixed up in the file so I wanted to ask an opinion before attempting it. Tagging the authors @Rémy Degenne and @Sébastien Gouëzel.

[Yaël Dillies]

I have been working on this file for the past month, so probably you can just skip it for now