Add filter/smoothing algorithms to plugin

[lehtonenp]

This pull request adds the following filtering functions to the plugin in a new group called Filtering

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

All of the above filtering functions were ran successfully in QGIS.

Note I put plausible values to minValue in the filter functions. If those need corrections, let me know.

[msmiyels]

@nmaarnio @lehtonenp

According to the notes made during coding: NLooks: int: >= 1 Damping Factor: Number: >= 0.0 Noise Mean: Number: any Noise Variance: Number: any

truncate and sigma are at least for the gaussian dependent from each other. But theoretically, truncate can be 0. If the resulting size of the window is too small (which it is every time if truncate equals zero), it throws an error anyway. sigma must be > 0.

Theoretically, noise_mean and noise_variance can have any value, but I would think restricting it to >= 0 would make sense, since I cannot really imagine what a negative noise should be or rather look like.

❓ Should we update the toolkit functions as well? E.g. noise min?

[nmaarnio]

Thanks! Yeah, I think it would be a good idea to update the minimum values / their documentation in the toolkit, but no big hurry with that.

[lehtonenp]

@nmaarnio, I have now set the minValue params to 0.