Fix element name convention

[aradi]

This allows the usage of element names matching the pattern [a-z][a-z0-9_] (ignoring case). This also would ensure, that each element name is a valid XML tag name (although not necessary for the application...)

[vanderhe]

@aradi Could we get a helpful error message for unsupported cases like Oß?

[bhourahine]

@aradi Could we get a helpful error message for unsupported cases like Oß?

So anything outside of ascii codes 48-57, 65-90, 95, 97-122 ? Unicode support seems permitted but not required: https://fortranwiki.org/fortran/files/character_handling_in_Fortran.html so any ideas what ß turns into with a non-supporting compiler?