Fix map_from_bases for single rv

[FriesischScott]

With this commit map_from_bases returns a Matrix regardless if one or more random variables are used. It also makes the nodes of the gauss quadrature a proper Matrix instead of an adjoint.

Afterwards #126 can be simplified and merged.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (3e0da86) 99.04% compared to head (37dc24b) 99.04%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #127 +/- ## ======================================= Coverage 99.04% 99.04% ======================================= Files 23 23 Lines 839 840 +1 ======================================= + Hits 831 832 +1 Misses 8 8 ``` | [Files Changed](https://app.codecov.io/gh/FriesischScott/UncertaintyQuantification.jl/pull/127?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/pcebases.jl](https://app.codecov.io/gh/FriesischScott/UncertaintyQuantification.jl/pull/127?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jasper+Behrensdorf#diff-c3JjL21vZGVscy9wY2UvcGNlYmFzZXMuamw=) | `100.00% <100.00%> (ø)` | | | [src/models/pce/polynomialchaosexpansion.jl](https://app.codecov.io/gh/FriesischScott/UncertaintyQuantification.jl/pull/127?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jasper+Behrensdorf#diff-c3JjL21vZGVscy9wY2UvcG9seW5vbWlhbGNoYW9zZXhwYW5zaW9uLmps) | `97.87% <100.00%> (+0.04%)` | :arrow_up: |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.