Fix mse for least squares PCE

[andreaperin]

The evaluation of the mse of PCE with "least squared" method was performed without the division by the number of samples.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (b3130d5) 99.06% compared to head (cb91530) 99.06%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #128 +/- ## ======================================= Coverage 99.06% 99.06% ======================================= Files 23 23 Lines 852 852 ======================================= Hits 844 844 Misses 8 8 ``` | [Files Changed](https://app.codecov.io/gh/FriesischScott/UncertaintyQuantification.jl/pull/128?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jasper+Behrensdorf) | Coverage Δ | | |---|---|---| | [src/models/pce/polynomialchaosexpansion.jl](https://app.codecov.io/gh/FriesischScott/UncertaintyQuantification.jl/pull/128?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jasper+Behrensdorf#diff-c3JjL21vZGVscy9wY2UvcG9seW5vbWlhbGNoYW9zZXhwYW5zaW9uLmps) | `98.30% <100.00%> (ø)` | |

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[FriesischScott]

Can you rebase you commit on master so the history is correct? The solver stuff was already merged in a previous PR.