Implement a square root grid

[timcallow]

This PR implements a square root grid as alternative to the current logarithmic grid. The idea is that this has many more points near the RH boundary, so higher values of lmax and nmax can be accessed.

[codecov[bot]]

Codecov Report

Merging #179 (9b127a4) into develop (f98caca) will decrease coverage by 6.01%. The diff coverage is 78.29%.

@@             Coverage Diff             @@
##           develop     #179      +/-   ##
===========================================
- Coverage    97.81%   91.81%   -6.01%     
===========================================
  Files           14       14              
  Lines         2247     2359     +112     
===========================================
- Hits          2198     2166      -32     
- Misses          49      193     +144     

Impacted Files	Coverage Δ
atoMEC/postprocess/pressure.py	9.47% <4.34%> (-88.09%)	:arrow_down:
atoMEC/mathtools.py	90.62% <46.66%> (-8.22%)	:arrow_down:
atoMEC/check_inputs.py	94.88% <75.00%> (-1.20%)	:arrow_down:
atoMEC/staticKS.py	95.55% <77.63%> (-3.13%)	:arrow_down:
atoMEC/postprocess/localization.py	93.96% <84.61%> (-1.45%)	:arrow_down:
atoMEC/numerov.py	87.61% <86.76%> (-8.46%)	:arrow_down:
atoMEC/models.py	91.97% <87.50%> (-4.89%)	:arrow_down:
atoMEC/__init__.py	99.04% <100.00%> (ø)
atoMEC/config.py	100.00% <100.00%> (ø)
atoMEC/convergence.py	100.00% <100.00%> (ø)
... and 1 more

[timcallow]

Codecov report is failing I think because pressure_test.py has split into pressure_test_log.py and pressure_test_sqrt.py, and codecov might not be able to handle discrepancies in test names between develop and new branches.

Since the tests are passing I am merging this PR and hopefully the codecov issue resolves itself. As can be seen from the attached screenshot, running the tests locally gives a coverage of 97%.