Optimization changes

[bclyons12]

These changes, and the ones in GGDUtils.jl, should be checked that they give the same results as before.

Timings for SynthDiag.compute_interferometer(ids)

• Initial: 17.734445 seconds (52.18 M allocations: 10.914 GiB, 12.23% gc time)
• With SynthDiag changes only: 13.543699 seconds (39.35 M allocations: 10.366 GiB, 10.35% gc time)
• With GGDUtils changes only: 10.897210 seconds (15.08 M allocations: 1.091 GiB, 2.48% gc time)
• With both: 8.018750 seconds (101.61 k allocations: 532.300 MiB, 1.89% gc time)

Some simple changes:

• Remove unnecessary array allocation
• Improve performance of _G function

Some complicated changes

• Avoid returning multiple-dispatched function calls (just seems like a bad idea to me)
• Simplify creation on anonymous functions
• Use let block to avoid captured variables

[orso82]

I can feel the memory taking a sigh of relief :)

[anchal-physics]

This PR lists important places where julia code can be improved in speed by ensuring that most of the allocation of memory for arrays and function calls is happening during compilation, so that during runtime the code executes fast.