Implement exponential cone constraints

[migarstka]

I think we should implement exponential cone constraints:
K_exp = {(x, y, z) | y > 0, y exp( x / y) <= z } U {(x, y, z) | x <= 0, y = 0, z >= 0}

After a quick look at [Parikh, Proximal Algorithms, p.184] it doesn't seem to be too difficult to implement. It would, however, allow us to solve a variety of new problems:

• exponential constraints t >= exp(x)
• logarithm constraints t <= log(x)
• entropy problems t <= - x log(x)
• logistic regression

for more examples:
https://yalmip.github.io/tutorial/exponentialcone/

https://docs.mosek.com/modeling-cookbook/expo.html

[migarstka]

implemented in 44cc8b3