alpha-2 release

Some of the bugs from the previous versions have now been fixed.

What's new?

snntorch

• SRM0 neuron model fix
• Reset now applies the threshold rather than '1'
• Reset by subtraction and reset to zero methods applied to both Stein and SRM0 neurons

snntorch.spikegen

• Delta modulation

snntorch.surrogate

• Optimized grad calculation

dev notes

• Travis-CI is no longer free. Replaced travis.yml with GH actions integration + tox