Change base time unit from ps to fs

[azonenberg]

This will be a major, wide-reaching refactor across huge amounts of the code, so should not be taken lightly.

Range should still be adequate: 2^64 fs is around five hours, which should be more than enough for any single scope acquisition.

But for higher speed serial data, it's possible that we're going to start seeing problems due to roundoff as we convert things to ps. The CDR PLL is already using floating point timestamps to work around this.

Discuss.

[azonenberg]

Required to support 80 Gsps scopes.