Split tpr header reading reading from tpr body

Split tpr header reading reading from tpr body

Split the low level functions for TPR file header and file body reading
into fully separate parts to allow reading the main part of the file
without having to read the header again.

Also gave the header datastructure a new name in line with naming
conventions and default initialized all fields.

Refs #2971

Change-Id: I110fb80cf19d9d2e59df1576e50c64806f532e00