Refactor H5Easy to use core.

[1uc]

Removes all custom implementations for serializing Eigen, XTensor, OpenCV, std::vector and uses HighFive core instead.

This allows us to remove most specialization, except the one for Eigen which serializes Matrix<double, 1, -1> as a one-dimensional array and reads it from one- or two-dimensional data.

[1uc]

Note that it's current based on top of #1011.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 86.78%. Comparing base (cb77ede) to head (ca6cba3).

Files	Patch %	Lines
include/highfive/h5easy_bits/H5Easy_scalar.hpp	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1016      +/-   ##
==========================================
+ Coverage   86.72%   86.78%   +0.05%     
==========================================
  Files         102      101       -1     
  Lines        6043     5964      -79     
==========================================
- Hits         5241     5176      -65     
+ Misses        802      788      -14     

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