Fix/instantaneous rate (#453)

* fixed deprecation warnings, replaced homogeneous_poisson_process with StationaryPoissonProcess

* added import for scipy.signal

* fixed size of output for instantaneous_rate, edited unittests accordingly

* test to check length of outputs

* added unittest to check consistency of rate times (should be multiple of sampling period), to get consistent ouput sizes, cutting of wings is done by fft convolve for center_kernel=True

* fixed pep8, added comments, restructure

* added include endpoint option

* added notes to docstrings, reformat docstrings and code, removed endpoint option

* fix docstrings

* restructured unit tests, added regression tests

* fixed pep8

* fixed indentation

* fixed wrong return