Add processing algorithms for filter/smoothing functions

[nmaarnio]

• [x] Focal filter
• [x] Gaussian filter
• [x] Mexican hat filter
• [x] Lee additive noise filter
• [x] Lee multiplicative noise filter
• [x] Lee additive multiplicative noise filter
• [x] Lee enhanced filter
• [x] Gamma filter
• [x] Frost filter
• [x] Kuan filter

[nmaarnio]

Update: CLI functions for the filters have been merged in EIS Toolkit (PR [#307])(https://github.com/GispoCoding/eis_toolkit/pull/307), so this task can be started.

If/when you have time @lehtonenp , feel free to take and assign this to yourself.