Incompatibility of sigpy dependency with scipy 1.14.0 and numpy 2.0

[schuenke]

The sigpy dependency is currently incompatible with the latest releases of scipy (v. 1.14.0) and numpy (v. 2.0). This leads to errors in PyPulseq when it's installed in a fresh environment. I already fixed this in sigpy (mikgroup/sigpy#143) and hope for a quick merge. If they merge it, no changes on our side should be necessary.

[wtclarke]

Given that sigpy doesn't seem well maintained currently, I've submitted #188 to remove the sigpy dependency where possible and then make it an optional dependency if you want to use the make_sigpy_pulse function. What do you think?