Udkm1dsim

What's Changed
* make conv_with_function static by dschick in https://github.com/dschick/udkm1Dsim/pull/116
* Update LLB.convert_caresian_to_polar() by Nilodirf in https://github.com/dschick/udkm1Dsim/pull/130
* fix issue98: recursive atom mixed by dschick in https://github.com/dschick/udkm1Dsim/pull/133
* Implement backside excitation by dschick in https://github.com/dschick/udkm1Dsim/pull/132
* Implement LLB simulations by dschick in https://github.com/dschick/udkm1Dsim/pull/126

New Contributors
* Nilodirf made their first contribution in https://github.com/dschick/udkm1Dsim/pull/130

**Full Changelog**: https://github.com/dschick/udkm1Dsim/compare/v1.5.6...v2.0.0

minor bug fixes
updates to documentation
use `sympy.lambdify` in generation and integration of temperature-dependent layer parameters

Small but important fix in the indexing of the initial conditions of the analytical phonon model.
Many thanks to Roman Shayduk!

Despite some bug fixes and improvements of the documentation, this release includes an analytical phonon model to simulate coherent acoustic phonon.

The complete udkm1Dsim has been implemented in Python from MATLAB.
New functionalities have been already added.