Just some minor performance improvements and warning fixes

[FlorianPfaff]

Hi,
since I'm currently experimenting with your code with spherical harmonics and made some minor improvements, I just thought I'd as well make them available (considering this github project is public).
Most changes are just fixes for Matlab editor warnings for reducing the use of defval. Some additional notes:

1. The code should be fully backwards compatible to yours. I added the possibility to put the files for LEGENDRE in the subfolder located where the xyz2plm.m file is located but set it to false by default.
2. I just put my name in "last modified by" to be consistent with the utilized convention, feel free copy the changes to your branch and put your name there.
3. I assume the datafit is the one available as part of slepian_bravo. I changed the function call because that makes it more pretty to eliminate defval statements over there as well.

Greetings,

Florian