v0.7.0: dev-0.7.0 (#81)
* new API :: device_type
* more constructors
* chebyshev_distenace
* stdc++fs
* COC
* deprecate <stdtensor>
* fix test
* support .at() for rank 0 tensor
* raw to flat
* flat_tensor::{rank,size,dims}
* simplify opencv example
* fix test for gcc