Add ULDM preset model

[AlexLaroche7]

• Added 'ULDM' preset model to preset_models. Uses WDMGeneral skeleton. Characteristic mass at which point CDM and ULDM differ and cutoff mass are both calculated within the function, can be changed depending on how you want to format it. The cutoff mass is given as a lambda function. I noticed that inputting a function for 'log_mlow' substantially increases runtime, but only up to a few minutes. ULDM particle mass and ULDM core-halo power law exponent are also arguments for the preset model.
• Included Zeta function in 'ULDM' halo model. This is relevant for the redshift dependence of the core radius. Important when creating halos near the cutoff mass.
• Minor change to test_uldm, can ignore.

[AlexLaroche7]

That should address both issues. Let me know if there is anything else, thanks for the feedback!