One-dimensional Helmholtz kernel

[ilfreddy]

In order to solve the Schrödinger Equation in 1d, we need to include the 1d Helmholtz kernel. That is simply a "Slater-type" function.

• Create class for Slater-type functions (bonus: we can do it for all dimensions)
  $$s(\mathbf{x}) = c e^{-a |\mathbf{x}-\mathbf{x}_0|}$$
• Implement the corresponding operator: a NS form which has $s(|x-y|)$ as a kernel
• Test

This operator does not require an expansion in a sum of Gaussians, but it is one component only. It is not singular either and its NS form should therefore decay pretty efficiently

[codecov]

Codecov Report

Attention: Patch coverage is 86.15385% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 65.80%. Comparing base (75b4ebe) to head (198a81b).

❗ Current head 198a81b differs from pull request most recent head 7f5d40b. Consider uploading reports for the commit 7f5d40b to get more accurate results

Files	Patch %	Lines
src/functions/Slater.h	35.71%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #237      +/-   ##
==========================================
+ Coverage   65.70%   65.80%   +0.09%     
==========================================
  Files         185      188       +3     
  Lines       10232    10299      +67     
==========================================
+ Hits         6723     6777      +54     
- Misses       3509     3522      +13     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.