Add MATLAB version 2018a support

[H0R5E]

This commit adds MATLAB 2018a support. This is an amalgamation of #109, #111 and #112, whilst reverting to the toolbox installation checking code used prior to 5acf452. The docs are also updated to clarify the legacy support situation.

Vanilla (MATLAB 2020a Windows):

>> dependencyCheck

WecOptTool Dependency Checker
-------------------------------

Required
--------
Optimization Toolbox:          Not Installed
NEMOH:                         Found

Optional
--------
Parallel Toolbox:              Not Installed
WAFO:                          Not found

Warning: Mandatory dependencies are missing! 
> In dependencyCheck (line 119) 

---

Without PCT installed (MATLAB 2020a Windows):

>> dependencyCheck

WecOptTool Dependency Checker
-------------------------------

Required
--------
Optimization Toolbox:          Found
NEMOH:                         Found

Optional
--------
Parallel Toolbox:              Not Installed
WAFO:                          Not found

Tests: test_results_serial.pdf

---

With PCT installed (MATLAB 2020a Windows):

>> dependencyCheck

WecOptTool Dependency Checker
-------------------------------

Required
--------
Optimization Toolbox:          Found
NEMOH:                         Found

Optional
--------
Parallel Toolbox:              Found
WAFO:                          Not found

Tests: test_results.pdf

---

@ryancoe, please test this on MATLAB 2018a @ssolson can you check this on Linux?

[ryancoe]

All passed in 2018a OSX test_results_serial.pdf test_results.pdf