Minor improvements

[maltezfaria]

Small fixes an improvements, including:

• Rename LagrangeMesh --> Mesh. No reason to call this LagrangeMesh, specially since that is no longer true.
• Rename AbstractPDE --> AbstractDifferentialOperator. These are not really equations, but linear operators.
• Fix typos in docstrings for e.g. Laplace and alike. We use the "positive definite" convention, where Laplace operator is $u \mapsto -\Delta u$
• Add tolerance to codecov failure
• Build documentation on GitHub runners

I did not split this into several PRs since the overhead of doing so seemed unnecessary.

[codecov]

Codecov Report

Attention: Patch coverage is 71.13402% with 28 lines in your changes missing coverage. Please review.

Project coverage is 66.00%. Comparing base (cc78dea) to head (c2e5cde).

Files with missing lines	Patch %	Lines
src/vdim.jl	0.00%	9 Missing ⚠️
src/api.jl	28.57%	5 Missing ⚠️
ext/IntiFMMLIB2DExt.jl	0.00%	4 Missing ⚠️
src/mesh.jl	86.66%	4 Missing ⚠️
src/kernels.jl	90.62%	3 Missing ⚠️
src/nystrom.jl	0.00%	2 Missing ⚠️
ext/IntiGmshExt.jl	50.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (cc78dea) and HEAD (c2e5cde). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (cc78dea)	HEAD (c2e5cde)
	2	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #101       +/-   ##
===========================================
- Coverage   80.03%   66.00%   -14.03%     
===========================================
  Files          24       24               
  Lines        3030     3015       -15     
===========================================
- Hits         2425     1990      -435     
- Misses        605     1025      +420     

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