New exact functions for exponential and logistic growth

[jamesmaino]

Also a new simple module for founder effects.

[rafaqz]

looks great. I can merge when it passes the tests. Then you can update the scripts to match this commit and push

[rafaqz]

Getting there! Maybe run ] test Dispersal locally and work through the errors.

You're getting errors like:

 Got exception UndefVarError(:SuitabilityExponentialGrowth) outside of a @test
  UndefVarError: SuitabilityExponentialGrowth not defined

in growth.jl tests, you probably renamed some growth model structs.

[coveralls]

Pull Request Test Coverage Report for Build 67

• 7 of 33 (21.21%) changed or added relevant lines in 4 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-4.3%) to 41.848%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/allee.jl	0	1	0.0%
src/local/inwards.jl	1	3	33.33%
src/local/outwards.jl	2	5	40.0%
src/growth.jl	4	24	16.67%
<!--	Total:	7	33	21.21%	-->

Files with Coverage Reduction	New Missed Lines	%
src/local/outwards.jl	2	22.22%
<!--	Total:	2		-->

Totals
Change from base Build 64:	-4.3%
Covered Lines:	77
Relevant Lines:	184

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 67

• 7 of 33 (21.21%) changed or added relevant lines in 4 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-4.3%) to 41.848%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/allee.jl	0	1	0.0%
src/local/inwards.jl	1	3	33.33%
src/local/outwards.jl	2	5	40.0%
src/growth.jl	4	24	16.67%
<!--	Total:	7	33	21.21%	-->

Files with Coverage Reduction	New Missed Lines	%
src/local/outwards.jl	2	22.22%
<!--	Total:	2		-->

Totals
Change from base Build 64:	-4.3%
Covered Lines:	77
Relevant Lines:	184

---

💛 - Coveralls

[rafaqz]

Merged! Awesome. Coveralls is grumpy we'll hae to add a few more tests.