Remove TABs from src

Will also make selftest to check that they aren't introduced

Use standard (>>, >-, >|) symbolic connectives for THEN*

This is done under src/, replacing Joe Hurd's earlier versions of the
same which were ++, >> and << respectively. His proofs also
occasionally rely on the fact that he made >> and >|
right-associative, which is a great idea. His symbols are inherently
bad but they clash with existing practice, and there seems to be a
good argument to trying to keep material in src reasonably
consistent. (Using >> for THEN1 instead of THEN is particularly
confusing given our current practice.)

Get core HOL to build with new definition of "by"

Progress with github issue #407

Added REAL_SIGMA_IMAGE_CROSS_SYM to real_sigmaTheory

Added REAL_SUM_IMAGE_IN_IF_ALT

Delete trailing whitespace in src/

Sum of a real-valued function over a set