Move ASCOT5 submodules to point to more recent versions

[bielsnohr]

There are a number of changes and bug fixes that have been brought into ASCOT5 since the ascot5.2 tag that Phaethon is currently pinned to. The major change is the use of momenta for particles instead of velocities. Whilst the direct benefit of this change is negligible for Phaethon itself, staying at the HEAD of the develop branch is desirable to keep up with updates.

This will also require ascot5-python to be put onto the master branch.

In both cases, the branches I am working on with minor modifications should be rebased onto the appropriate branch mentioned above.

The use of momenta will also require the methods that calculate particle energies to be modified.