Added utility functions for loads

[ccaprani]

• Functions for a nice interface for loads, such as add_udl and so on;
• Added an add_LM function so that load matrices can be added, making it easier to do superposition
• Changed the introductory examples 1 and 2 to reflect the new loads interface
• Added more examples showing a subframe and postensioning using the add_LM
• Added a new test of the utility functions, and general docs updates.

[codecov-commenter]

Codecov Report

Merging #38 (bcb0b7d) into main (164a0f0) will increase coverage by 0.11%.
The diff coverage is 86.53%.

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@@            Coverage Diff             @@
##             main      #38      +/-   ##
==========================================
+ Coverage   79.98%   80.09%   +0.11%     
==========================================
  Files          12       12              
  Lines        1179     1221      +42     
==========================================
+ Hits          943      978      +35     
- Misses        236      243       +7     

Impacted Files	Coverage Δ
pycba/load.py	89.72% <40.00%> (-2.12%)	⬇️
pycba/beam.py	84.92% <90.90%> (-0.33%)	⬇️
pycba/analysis.py	78.37% <100.00%> (+3.12%)	⬆️
pycba/results.py	99.21% <100.00%> (ø)
tests/test_basic.py	100.00% <100.00%> (ø)

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