Fix rotate and poisson

[m-reuter]

1. when computing a function with rotated gradients, the problem was singular due to the degree of freedom to shift the function (addition of constant). This can be fixed by specifying an arbitrary dirichlet boundary condition at a vertex (here: 0, 0.0).
2. in poisson, when a function h was passed in connection with a dirichlet boundary condition, the function h needs to be a 2D matrix with a single column instead of a 1-dim vector. So we add a new axis.

[codecov]

Codecov Report

Attention: Patch coverage is 20.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 34.89%. Comparing base (109511c) to head (ed4e433).
Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
lapy/diffgeo.py	0.00%	2 Missing ⚠️
lapy/plot.py	0.00%	1 Missing ⚠️
lapy/solver.py	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #69      +/-   ##
==========================================
- Coverage   34.89%   34.89%   -0.01%     
==========================================
  Files          14       14              
  Lines        2367     2370       +3     
  Branches      299      286      -13     
==========================================
+ Hits          826      827       +1     
- Misses       1457     1458       +1     
- Partials       84       85       +1     

Flag	Coverage Δ
unittests	34.89% <20.00%> (-0.01%)	⬇️

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