Univariate Normals

[filtron]

Fixes #101 and #102

What has been done:

• added constructor for Normal(::Number, ::Number)
• added alias UvNormal{T} for Normal{T,T,T} where {T<:Number}
• added some missing methods for UvNormal
• made some AbstractNormal methods more generic, so they work for UvNormal as well.

Some things to consider:

• Should UvNormal store the covariance as a real type? i.e. should it be UvNormal{T,V} for Normal{T,T,V} where {T<:Number,V<:real(T)}. In this case, the first point is irrelevant?

Future work:

• add scalar to vector, scalar to scalar, vector to scalar constructors for AffineMap so that corresponding kernels can be constructed.
• add constructors to NormalKernel and DiracKernel for the aforementioned affine maps.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 20.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 91.93%. Comparing base (44e6544) to head (7bf0ecd). Report is 7 commits behind head on main.

Files	Patch %	Lines
src/distributions/uvnormal.jl	0.00%	7 Missing :warning:
src/covariance_parameter.jl	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #103 +/- ## ========================================== - Coverage 95.41% 91.93% -3.49% ========================================== Files 16 19 +3 Lines 393 409 +16 ========================================== + Hits 375 376 +1 - Misses 18 33 +15 ```

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[filtron]

I had one new comment. In addition, as you say that this fixes #101, should we maybe add a test that ensures that this is acutally the case? Otherwise if you didn't check that this actually works now, I would suggest not closing #101 yet.

I added a test to make sure you can add constructor methods for normals with PSDmatrix now.