A few more or less obvious fixes for speed of SasView startup

[rozyczko]

From >30 sec down to ~5 sec.
numba related fixes suggested by @pkienzle + minor other import shuffling as suggested by startup profiles.

[krzywon]

Does what it says it does. I measured the time to open the main window and it decreased from 21.5 seconds running the main branch to 11.1 seconds for this one.

[gonzalezma]

More modest gain in my Windows machine (from ~30" to ~18"), but still an improvement.

[pkienzle]

Maybe src/sas/sascalc/pr/p_invertor.py as well?

[rozyczko]

Maybe src/sas/sascalc/pr/p_invertor.py as well?

In the startup profiling (Win10, dev build), importing the inversion perspective takes less than 1%. I'll add it anyway.

Other external imports, e.g. from bumps import options take significantly more time (>12%). I'm looking at making this import lazy as well.

[rozyczko]

importing bumps options can't be made lazy, since we need to display the default bumps minimizer name, meaning importing and querying the module at start time.
We could hard-code the initial value to LM but this is not pretty.
Leaving as is then.

[wpotrzebowski]

The changes to the code seem harmless and it works fine on Mac