Make single and double precision restarts compatible with either version of the code

[pcarruscag]

Proposed Changes

The 4th int of the header was written as 0 and never read, it now holds the size in bytes of the floating point data, so single and double precision builds can read each other's restart files. Files written before this field existed have a 0 there and were always double.

The MPI-IO read paths were hardcoded to MPI_DOUBLE, which is redefined to MPI_FLOAT in single precision builds and was therefore consistent within a build but not across builds. They now describe the payload as blocks of bytes of the size given by the header, and convert to the precision of the build when the two differ.

PR Checklist

• I am submitting my contribution to the develop branch.
• My contribution generates no new compiler warnings (try with --warnlevel=3 when using meson).
• My contribution is commented and consistent with SU2 style (https://su2code.github.io/docs_v7/Style-Guide/).
• I used the pre-commit hook to prevent dirty commits and used pre-commit run --all to format old commits.
• I have added a test case that demonstrates my contribution, if necessary.
• I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp), if necessary.