Add a means table print out to umxSummary

[tbates]

It would be useful for users to be able to get a table of means (just as they can request genetic correlations with showRg= TRUE now)

for twins, where we now print:

	a1	c1	e1
wt1	-0.83	.	-0.56

We could do:

	a1	c1	e1	mean
wt1	-0.83	.	-0.56	75.3

[mcneale]

Yep. I think printing zero or .00 would be much better than a . which some software (cough, SAS) might interpret as a missing value.

[tbates]

Interesting: I wrote a special print function to suppress 0.00 to make (esp. larger ACE tables full of zeros) tables easier to read.

I'll add a TODO to add an option to set this place-keeper to whatever symbol the user desires.

factor analysis has it's own print method which allows suppressing small values, and they just print blank. so could follow that model.

[tbates]

this is now in place for umxACE, and some other models.
means tables also can be report="html" tables for copying to a word processor.