Add inverse function

[sandrlom]

First, thank you for providing this framework. This PR proposes the inv function which computes the inverse of an invertible matrix. Let me know if you'd like something changed.

[jonasrauber]

@sandrlom Thanks! Looks great so far. The only problem is that we still need proper tests. They currently fail because inv gets called with 1-dimensional array/tensor. Please include a non-trivial example (not just an identity-matrix).

[jonasrauber]

@sandrlom Thanks! Tests fail because of formatting issues (please reformat the code using black / manually fix the issues reported by flake8).

Once tests pass, let's get this merged :)

[codecov[bot]]

Codecov Report

Merging #48 (1578b94) into master (e593fda) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #48   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           16        16           
  Lines         1860      1870   +10     
=========================================
+ Hits          1860      1870   +10     

Impacted Files	Coverage Δ
eagerpy/tensor/tensor.py	100.00% <ø> (ø)
eagerpy/framework.py	100.00% <100.00%> (ø)
eagerpy/tensor/jax.py	100.00% <100.00%> (ø)
eagerpy/tensor/numpy.py	100.00% <100.00%> (ø)
eagerpy/tensor/pytorch.py	100.00% <100.00%> (ø)
eagerpy/tensor/tensorflow.py	100.00% <100.00%> (ø)

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[jonasrauber]

Thanks @sandrlom