Add detailed explanation on the typeOfFE::Data

[dohyun-cse]

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I want to add a new element to FreeFem. Defining interpolation points and associated vertex/edge/element is quite straightforward from the current documentation.
However, the definition of typeOfFE::Data is not well documented.
According to the documentation, it should be 5*NbDoF + N where the first 5 NbDoF elements are
(1) (NbDoF) the support of the node of the df (maybe for unique id?)
(2) (NbDoF) the number of the df on the node
(3) (NbDoF) the node of the df (not sure what this means)
(4) (NbDoF) the df come from which FE (also not sure)
(5) (NbDoF) which are the df on sub FE (...)
(6) (N) no explanation

Also, I found that, in the source code, P1Lagrange and P2Lagrange have 3*5 + 3 and 6*5 + 3, respectively.
This does not match with the documentation since N=1 as they are scalar spaces.

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Can anyone add more detailed explanation about the typeOfFE::Data

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